You cant intiate object like this
//Create NumberPickers
NumberPicker numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1);
NumberPicker numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2);
after onCreate only you layout loads so you dont have you object yet initialized. You can initlize you object in onCreate after
setContentView(R.layout.activity_main);
For example
// init variable
NumberPicker numberPicker1 = null;
NumberPicker numberPicker2 = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
setContentView(R.layout.activity_main);
// After here only your Activity gets the layout objects
numberPicker1 = (NumberPicker) findViewById(R.id.numberPicker1);
numberPicker2 = (NumberPicker) findViewById(R.id.numberPicker2);
}