App keeps crashing.

                    Here is my MainActivity.java file:

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v7.app.ActionBarActivity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

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


        //Declare our View variables
        final TextView answerLabel = (TextView) findViewById(R.id.textView1);
        Button getAnswerButton = (Button) findViewById(R.id.button1);

        getAnswerButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // The button was clicked, so update the answer label with an answer
                String answer = "Yes";
                answerLabel.setText(answer);

            }
        });

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }

}

                    When going through the debugger line by line it fails in the ThreadGroup.class

/**
     * Handles uncaught exceptions. Any uncaught exception in any {@code Thread}
     * is forwarded to the thread's {@code ThreadGroup} by invoking this
     * method.
     *
     * <p>New code should use {@link Thread#setUncaughtExceptionHandler} instead of thread groups.
     *
     * @param t the Thread that terminated with an uncaught exception
     * @param e the uncaught exception itself
     */
    public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);                                      <--- This is the line it fails at
        } else if (Thread.getDefaultUncaughtExceptionHandler() != null) {
            // TODO The spec is unclear regarding this. What do we do?
            Thread.getDefaultUncaughtExceptionHandler().uncaughtException(t, e);
        } else if (!(e instanceof ThreadDeath)) {
            // No parent group, has to be 'system' Thread Group
            e.printStackTrace(System.err);
        }
    }

    /**
     * Called by the Thread constructor.
     */