Android guessing game? How to change the random number when user gets it right and solve crash fix when user enters nothing?

I'm trying to create a random number generator on Android Studio using JScript. The app works but it crashes when the user enters nothing and also it doesn't change the random number until the user restarts the app. I've tried the code below :

    public class MainActivity extends AppCompatActivity {
        int randomNumber;
        public void checkGuess(View view){
            EditText guessedNumber = (EditText) findViewById(R.id.guessedNumber);
            String guessedNumberString = guessedNumber.getText().toString();
            int guessedNumberInt = Integer.parseInt(guessedNumberString);
            String message = "";

            if (guessedNumberInt>randomNumber){
                message = "Too large!";
            }else if(guessedNumberInt<randomNumber){
                message = "Too small!";
            }else {
                message = "correct!";
            }

            Toast.makeText(getApplicationContext(), message, Toast.LENGTH_LONG).show();
}


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

        Random randomGenerator = new Random();
        int randomNumber = randomGenerator.nextInt(21);
    }
}

What I should do to change the random number when user guesses it right and how to fix the app crash when user enters nothing?