android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

Question

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

public class MainActivity extends AppCompatActivity {

    Button yrebtn;
    Button yrebtn1;
    Button yrebtn2;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        yrebtn = (Button) findViewById(R.id.yrebtn);
        yrebtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, MainActivity1.class);
                startActivity(intent);
            }
        });

        yrebtn1 = (Button) findViewById(R.id.yrebtn1);
        yrebtn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, MainActivity2.class);
                startActivity(intent);
            }
        });

        yrebtn2 = (Button) findViewById(R.id.yrebtn2);
        yrebtn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, MainActivity3.class);
                startActivity(intent);
            }
        });
    }
}