Attempt to invoke virtual method / null

Question

Im a bit lost here, I'm getting the error:

Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference

This is the code that matters:

public class MainActivity extends AppCompatActivity {

    SeekBar chestValue;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        chestValue = (SeekBar)findViewById(R.id.chestValueSlider);

}

The line "SeekBar chestValue;" is whats giving the error according to the stacktrace. Any suggestions?