Button doesn't work on real phone, but works on android studio emulator

Question

Using Build->Build APK, I tried to install my app into my phone. Everything works fine including the button, on the emulator.

However, when I try using it on my phone, pressing the button crashes my phone. Floating action button works, but not

I tried commenting out the startActivity functions that are linked to the button, but it still crashes. Any possible reasons why?

Sorry if it is a bit vague, but I am not sure what the root of the problem is.

I tried setting backToMainScreen method to nothing, but the app still crashes, so I don't think it is a problem with the java code.

One of the pages that caused problem

public class AddExercise extends AppCompatActivity {

private ArrayList<Exercise> exList;

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.add_exercise);
    exList =  MainActivity.loadData();
}

void backToMainScreen(View target){
    Intent myIntent = new Intent(getApplicationContext(),    MainActivity.class);
    MainActivity.saveData(exList);

    startActivity(myIntent);
    finish();
}

void addExercise(View target){

    EditText etNewEntry = (EditText) findViewById(R.id.newEntry);
    String newEntry = etNewEntry.getText().toString();
    if (!newEntry.isEmpty()) {
        Exercise newExercise = new Exercise(newEntry);
        exList.add(newExercise);
    }

    backToMainScreen(target);

}


}

Here is the XML code for that page

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/popupWindow">

<TextView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:text="@string/exInput"
    android:layout_above="@+id/newEntry"
    android:gravity="center"/>

<EditText
    android:layout_width="200sp"
    android:layout_height="wrap_content"
    android:id="@+id/newEntry"

    android:layout_above="@+id/buttons"
    android:layout_alignLeft="@+id/buttons"
    android:layout_alignStart="@+id/buttons"
   android:inputType="textAutoComplete"/>
<LinearLayout
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/buttons"
    android:layout_centerInParent="true"
    android:orientation="horizontal">
    <Button
        android:layout_width="100sp"
        android:layout_height="wrap_content"
        android:id="@+id/ok"
        android:text="@string/ok"
        android:onClick="addExercise"
        />
    <Button
        android:layout_width="100sp"
        android:layout_height="wrap_content"
        android:id="@+id/cancel"
        android:text="@string/cancel"
        android:onClick="backToMainScreen"
        />

</LinearLayout>


</RelativeLayout>

Answer 1

In order for android:onClick to work your target function should be public .

public void backToMainScreen(View target){}

public void addExercise(View target){}

Answer 2

In order for this to work, the method must be public and accept a View as its only parameter. Please make your methods public in your code for the button click.

Read the documentation https://developer.android.com/reference/android/widget/Button.html