why does my app crashes when i try adding a radio button to the radio group?

Question

i have a problem when trying to add a programmatically defined radio button to a radio group.

so i have a method that when is called inflates a dialog, this dialog contains within it a list of subjects displayed as a list of checkBox. since i can't predefine how many subjects are needed in my radio group i decided to add them programmatically using the method mentioned earlier. for some reason the app keep on crashing every time i open the dialog. with the break point being when the radio button gets added to the radio group. here is a copy of the fragment responsible of calling the method (as well as the method called):

public class addPaymentFragment extends Fragment {

    private RadioGroup radioGroup;
    private RadioButton radioButton;
    private SchoolViewModel schoolViewModel;
    private NumberPicker numberPicker;
    private Button btn;
    private Subject subject;
    private Dialog dialog;
    private RadioGroup radioGroup0;

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        View v = inflater.inflate(R.layout.fragment_add_payment, container, false);
        btn = v.findViewById(R.id.btn_add_payment);
        radioGroup0 = v.findViewById(R.id.rg_select_subject);
        radioGroup = v.findViewById(R.id.rg_add_payment);
        numberPicker = v.findViewById(R.id.numberPicker);
        numberPicker.setMinValue(0);
        numberPicker.setMaxValue(10);
        schoolViewModel = new ViewModelProvider(this).get(SchoolViewModel.class);
        radioGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(RadioGroup radioGroup, int i) {
                schoolViewModel.getSingleTeacherWithSubjects(i).observe(getViewLifecycleOwner(), new Observer<TeacherWithSubjects>() {
                    @Override
                    public void onChanged(TeacherWithSubjects teacherWithSubjects) {
                        if (teacherWithSubjects.subjects.size() > 1) {
                            openDialog(teacherWithSubjects.subjects,radioGroup0);
                        } else {
                            subject = teacherWithSubjects.subjects.get(0);
                        }
                    }
                });
            }
        });

        schoolViewModel.getAllTeachers().observe(getViewLifecycleOwner(), new Observer<List<Teacher>>() {
            @Override
            public void onChanged(List<Teacher> teachers) {
                addRadioButtons(teachers, radioGroup);
                btn.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Toast.makeText(getContext(), teachers.get(0).getTeacherName() + "", Toast.LENGTH_SHORT).show();
                    }
                });
            }
        });

        return v;
    }

    private void addRadioButtons(List<Teacher> teachers, RadioGroup radioGroup) {
        for (Teacher i : teachers) {
            //instantiate...
            RadioButton radioButton = new RadioButton(getContext());
            //set the values that you would otherwise hardcode in the xml...
            RadioGroup.LayoutParams params = new RadioGroup.LayoutParams(RadioGroup.LayoutParams.WRAP_CONTENT,
                    RadioGroup.LayoutParams.WRAP_CONTENT);
            params.bottomMargin = 25;
            params.leftMargin = 20;
            params.rightMargin = 20;
            params.topMargin = 20;
            radioButton.setLayoutParams(params);
            //label the button...
            radioButton.setBackground(new ColorDrawable(Color.TRANSPARENT));
            radioButton.setMinWidth(250);
            radioButton.setBackgroundResource(R.drawable.custom_checkbox);
            radioButton.setElevation(16);
            radioButton.setGravity(Gravity.CENTER);
            radioButton.setText(i.getTeacherName());
            radioButton.setPadding(50, 100, 50, 100);
            radioButton.setButtonDrawable(R.drawable.custom_checkbox);
            radioButton.setId(i.getTeacherId());
            //add it to the group.
            radioGroup.addView(radioButton);
        }
    }

    void openDialog(List<Subject> subjects,RadioGroup radioGroup) {
        dialog = new Dialog(getContext());
        radioGroup = dialog.findViewById(R.id.rg_select_subject);
        dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
        dialog.setContentView(R.layout.choose_teached_subject_dialog);
        dialog.setCancelable(false);
        Button confirmSubject = dialog.findViewById(R.id.btn_choose_subject);
        Button cancelSubject = dialog.findViewById(R.id.btn_dont_choose_subject);
        //populate the checkBox
        for (int i = 0; i<subjects.size(); i++){
            RadioButton radioButton1 = new RadioButton(dialog.getContext());
            RadioGroup.LayoutParams params = new RadioGroup.LayoutParams(RadioGroup.LayoutParams.WRAP_CONTENT,
                    RadioGroup.LayoutParams.WRAP_CONTENT);
            params.bottomMargin = 25;
            params.leftMargin = 20;
            params.rightMargin = 20;
            params.topMargin = 20;
            radioButton1.setLayoutParams(params);
            radioButton1.setBackground(new ColorDrawable(Color.TRANSPARENT));
            radioButton1.setMinWidth(250);
            radioButton1.setBackgroundResource(R.drawable.custom_checkbox);
            radioButton1.setElevation(16);
            radioButton1.setGravity(Gravity.CENTER);
            radioButton1.setId(i);
            radioButton1.setText(subjects.get(i).getSubjectName());
            radioButton1.setPadding(50, 100, 50, 100);
            radioButton1.setButtonDrawable(R.drawable.custom_checkbox);
            //add it to the group.
            radioGroup.addView(radioButton1);
        }
        confirmSubject.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dialog.dismiss();
                Toast.makeText(getContext(), "Select a subject!", Toast.LENGTH_SHORT).show();
            }
        });
        cancelSubject.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                dialog.dismiss();
            }
        });

        dialog.getWindow().setLayout(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.WRAP_CONTENT);
        dialog.getWindow().setBackgroundDrawable(new ColorDrawable(Color.TRANSPARENT));
        dialog.getWindow().getAttributes().windowAnimations = R.style.DialogAnimation;
        dialog.getWindow().setGravity(Gravity.CENTER);
        dialog.show();
    }
}

and here is the code to the corresponding XML file:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".addPaymentFragment">


    <HorizontalScrollView
        android:id="@+id/rv_add_payment"
        android:layout_width="match_parent"
        android:layout_height="146dp"
        android:orientation="horizontal"
        android:scrollbars="none"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <RadioGroup
            android:id="@+id/rg_add_payment"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"></RadioGroup>
    </HorizontalScrollView>

    <NumberPicker
        android:id="@+id/numberPicker"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:theme="@style/numberPickerTheme"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Heures Enseignées :"
        android:textColor="#000"
        android:textSize="28sp"
        app:fontFamily="@font/k2d_semibold"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/rv_add_payment" />

    <ImageView
        android:id="@+id/imageView2"
        android:layout_width="36dp"
        android:layout_height="36dp"
        android:layout_marginEnd="10dp"
        android:src="@drawable/ic_baseline_play_arrow_24"
        app:layout_constraintBottom_toBottomOf="@+id/numberPicker"
        app:layout_constraintEnd_toStartOf="@+id/numberPicker"
        app:layout_constraintTop_toTopOf="@+id/numberPicker" />

    <LinearLayout
        android:id="@+id/linearLayout"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="50dp"
        android:gravity="center"
        android:orientation="horizontal"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/numberPicker">

        <TextView
            android:id="@+id/textView6"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Total: "
            android:textColor="#000"
            android:textSize="28sp"
            app:fontFamily="@font/k2d_semibold" />

        <TextView
            android:id="@+id/textView8"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="0DA"
            android:textColor="#000"
            android:textSize="28sp"
            app:fontFamily="@font/k2d_semibold" />
    </LinearLayout>

    <android.widget.Button
        android:id="@+id/btn_add_payment"
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginStart="20dp"
        android:background="@drawable/rounded_button"
        android:text="add"
        android:textColor="@color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/linearLayout" />

    <android.widget.Button
        android:layout_width="100dp"
        android:layout_height="100dp"
        android:layout_marginEnd="20dp"
        android:background="@drawable/rounded_button"
        android:text="cancel"
        android:textColor="@color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/linearLayout" />


</androidx.constraintlayout.widget.ConstraintLayout>

and here is a copy of the XML file of the dialog called:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_marginStart="20dp"
    android:layout_marginTop="20dp"
    android:layout_marginEnd="20dp"
    android:layout_marginBottom="20dp"
    android:background="@drawable/dialog_background"
    android:elevation="16dp">

    <TextView
        android:id="@+id/choose_subject_tv"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        android:fontFamily="@font/k2d_semibold"
        android:text="choose teached subject :"
        android:textColor="@color/black"
        android:textSize="30dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <HorizontalScrollView
        android:id="@+id/horizontalScrollView"
        android:layout_width="match_parent"
        android:layout_height="146dp"
        android:orientation="horizontal"
        android:scrollbars="none"
        android:layout_marginTop="20dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/choose_subject_tv">

        <RadioGroup
            android:id="@+id/rg_select_subject"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"></RadioGroup>

    </HorizontalScrollView>

    <android.widget.Button
        android:id="@+id/btn_choose_subject"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="50dp"
        android:layout_marginTop="20dp"
        android:layout_marginBottom="40dp"
        android:background="@drawable/custom_button"
        android:fontFamily="@font/k2d_semibold"
        android:text="Confirm"
        android:textAllCaps="false"
        android:textColor="@color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/horizontalScrollView" />

    <android.widget.Button
        android:id="@+id/btn_dont_choose_subject"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="50dp"
        android:layout_marginBottom="40dp"
        android:layout_marginTop="20dp"
        android:background="@drawable/custom_button"
        android:fontFamily="@font/k2d_semibold"
        android:text="Cancel"
        android:textAllCaps="false"
        android:textColor="@color/white"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/horizontalScrollView" />

</androidx.constraintlayout.widget.ConstraintLayout>

and here is the message shown in the logcat:

    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.RadioGroup.addView(android.view.View)' on a null object reference
        at com.example.ecolemathphysique.addPaymentFragment.openDialog(addPaymentFragment.java:142)
        at com.example.ecolemathphysique.addPaymentFragment$1$1.onChanged(addPaymentFragment.java:62)
        at com.example.ecolemathphysique.addPaymentFragment$1$1.onChanged(addPaymentFragment.java:58)

i apologize in advance if there is any mistakes in my question, am a bit new to asking questions here so please take it easy on me and thanks in advance

Answer 1

so i have managed to find the problem which was i called radioGroup = dialog.findViewById(R.id.rg_select_subject); before finishing setting up the dialog, which mean that the radioGroup object was never assigned. here is how the related bloc looks like after fixing it:

 void openDialog(List<Subject> subjects,RadioGroup radioGroup) {
    dialog = new Dialog(getContext());
    dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
    dialog.setContentView(R.layout.choose_teached_subject_dialog);
    dialog.setCancelable(false);
    radioGroup = dialog.findViewById(R.id.rg_select_subject);
    Button confirmSubject = dialog.findViewById(R.id.btn_choose_subject);
    Button cancelSubject = dialog.findViewById(R.id.btn_dont_choose_subject);