Why does the Android Studio logcat show this error when the user presses the button?

Hi, whilst attempting to add functionality to a button in Android Studio, the following error is shown in the logcat:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

I am trying to get the button to direct the user to another activity when clicked. This is the java code that I have produced in order to add this function to the button:

Button physics_button = (Button) findViewById(R.id.physbutton);
physics_button.setOnClickListener(new View.OnClickListener(){
    public void onClick(View view){
        Intent intent = new Intent (MainActivity.this, RevisePhysics.class);
        startActivity(intent);
    }
});

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The button ID has been made the same as the ID within the ‘MainActivity’ XML file:

<Button
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:text="Physics"
    android:id="@+id/physbutton"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true"
    android:textSize="50dp"
    android:background="#82020202"
    android:textColor="#ffffff"
    android:fontFamily="Open Sans"
    android:onClick="onClick"/>

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The activity ‘RevisePhysics’ has been created:
 So I don’t understand why the error shown is stating the app is attempting to call a null object reference.
This button previously worked until I added content to the activity ‘RevisePhysics’ which, to my knowledge, shouldn’t affect the functionality of the button on ‘Main Activity’.
Thank you in advance for any replies.