Android Studio 4.0.1 GridLayout crashes the app

Question

I was learning how to take a grid and find some elements from the grid, but my app was crashing when I was looking to find the grid by id, so I decided to create a test app that would look for the grid right from the start and this app was crashing too. Note that at the testapp I introduced the legacy libraries to make sure that I don't miss something.
This is xml code for the view

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.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=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.264" />

    <android.support.v7.widget.GridLayout
        android:id="@+id/grid"
        android:layout_width="409dp"
        android:layout_height="354dp"
        android:layout_marginEnd="1dp"
        android:layout_marginStart="1dp"
        android:layout_marginTop="1dp"
        app:columnCount="3"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:rowCount="3">

    </android.support.v7.widget.GridLayout>

</android.support.constraint.ConstraintLayout>
 

and this is the mainActivity the crash happens at the "GridLayout test" invocation

package com.example.test2;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.GridLayout;

public class MainActivity extends AppCompatActivity {

    GridLayout test =(GridLayout) findViewById(R.id.grid);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

This is the Error message that I get

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.test2, PID: 11311
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.test2/com.example.test2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.support.v7.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:182)
        at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520)
        at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:191)
        at com.example.test2.MainActivity.<init>(MainActivity.java:9)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2669)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

Any help on how to fix this issue would be appreciated and some detailed explanation why it happens , thank you .

Answer 1

change your code from:

GridLayout test =(GridLayout) findViewById(R.id.grid);

to:

androidx.gridlayout.widget.GridLayout gridLayout = findViewById(R.id.gridLayout);

Answer 2

You are trying to get id before activity knows about its xml layout. You need to put get id line in the onCreate method as below

public class MainActivity extends AppCompatActivity {

    private GridLayout test;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        test =(GridLayout) findViewById(R.id.grid);
   }
}

Answer 3

For Android Studio 4.0 and above,

use import androidx.gridlayout.widget.GridLayout instead of import android.widget.GridLayout;