title: - java.lang.NullPointerException - setText on null object reference
body: <p>This is what I'm trying to do for several hours:
  I've got a MainActivity.java file (listing below) and a fragment_start.xml file with a start button. Tapping the start-button should display the activity_main.xml file with points-/round- and countdown-Textviews. It doesn't work and this is what is happening:</p>
  
  <p>The logcat tells me:
  <strong>PID: 1240 java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference</strong></p>
  
  <p>The emulator displays: <strong>Unfortunately, GAME has stopped.</strong></p>
  
  <p>Necessary to mention that I'm rather new in programming?</p>
  
  <p>Thanks for any advice!</p>
  
  <pre><code>import android.app.Activity;
  import android.os.Bundle;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.TextView;
  
  
  public class MainActivity extends Activity implements View.OnClickListener {
  
  private int points;
  private int round;
  private int countdown;
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_main);
      showStartFragment();
  }
  
  private void newGame () {
      points=0;
      round=1;
      initRound();
  }
  
  private void initRound() {
      countdown = 10;
      update();
  }
  
  private void update () {
      fillTextView(R.id.points, Integer.toString(points));
      fillTextView(R.id.round, Integer.toString(round));
      fillTextView(R.id.countdown, Integer.toString(countdown * 1000));
  }
  
  private void fillTextView (int id, String text) {
      TextView tv = (TextView) findViewById(id);
      tv.setText(text);
  }
  
  private void showStartFragment() {
      ViewGroup container = (ViewGroup) findViewById(R.id.container);
      container.removeAllViews();
      container.addView(
              getLayoutInflater().inflate(R.layout.fragment_start, null) );
      container.findViewById(R.id.start).setOnClickListener(this);
  }
  
  @Override
  public void onClick(View view) {
      if(view.getId() == R.id.start) {
          startGame();
      }
  }
  
  public void startGame() {
      newGame();
  }
  }
  </code></pre>