Fragment im Fragment erzeugt - Fehlermeldung beim Restart

public class Uebungen extends Fragment {

	String[] kapitel = new String[] { "Thema1", "Thema2",
			"Thema3" };
	ListView listView;

	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
			Bundle savedInstanceState) {

		View view = inflater.inflate(R.layout.u_listview, container, false);
		listView = (ListView) view.findViewById(R.id.lv1);
		ArrayAdapter<String> adapter = new ArrayAdapter<String>(
				this.getActivity(), android.R.layout.simple_list_item_1,
				kapitel);

		listView.setAdapter(adapter);

		listView.setOnItemClickListener(new OnItemClickListener() {

			@Override
			public void onItemClick(AdapterView<?> parent, View view,
					int position, long id) {
				fmAendern((byte) position);
			}
		});

		return view;

	}

	[B]public void fmAendern(byte kapitelnummer) {
		Fragment fragment = new Training(kapitelnummer);

		FragmentManager fm = getFragmentManager();
		fm.beginTransaction().replace(R.id.content_frame, fragment).commit();[/B]
	}

}

public class MainActivity extends Activity {

	private DrawerLayout mDrawerLayout;
	private ListView mDrawerList;
	private ActionBarDrawerToggle mDrawerToggle;

	private CharSequence mDrawerTitle;
	private CharSequence mTitle;
	CustomDrawerAdapter adapter;

	List<DrawerItem> dataList;

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

	
		dataList = new ArrayList<DrawerItem>();
		mTitle = mDrawerTitle = getTitle();
		mDrawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
		mDrawerList = (ListView) findViewById(R.id.left_drawer);

		mDrawerLayout.setDrawerShadow(R.drawable.drawer_shadow,
				GravityCompat.START);

	
		dataList.add(new DrawerItem("Startseite", R.drawable.ic_start)); 
		dataList.add(new DrawerItem("Thema1", 
				R.drawable.ic_sinn_und_zweck));
		dataList.add(new DrawerItem("Thema2", R.drawable.ic_einstieg)); 
		

		adapter = new CustomDrawerAdapter(this, R.layout.custom_drawer_item,
				dataList);

		mDrawerList.setAdapter(adapter);

		mDrawerList.setOnItemClickListener(new DrawerItemClickListener());

		getActionBar().setDisplayHomeAsUpEnabled(true);
		getActionBar().setHomeButtonEnabled(true);

		mDrawerToggle = new ActionBarDrawerToggle(this, mDrawerLayout,
				R.drawable.ic_drawer, R.string.drawer_open,
				R.string.drawer_close) {

			public void onDrawerClosed(View view) {
				getActionBar().setTitle(mTitle);
        invalidateOptionsMenu();
						
			}

			public void onDrawerOpened(View drawerView) {
        getActionBar().setTitle(mDrawerTitle);
				invalidateOptionsMenu(); 
											

			}
		};

		mDrawerLayout.setDrawerListener(mDrawerToggle);
			if (savedInstanceState != null) {
			return;
		} else {
			SelectItem(0);
		}
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

	public void SelectItem(int possition) {

		Fragment fragment = null;

		switch (possition) {
		case 0:
			fragment = new FragmentStartSeite();
    	break;
    case 1:
			fragment = new Kapitel1();
    	break;
    case 2:
			fragment = new Uebungen();
      break;
      ...
   	default:
			break;
		}

		FragmentManager frgManager = getFragmentManager();
		frgManager.beginTransaction().replace(R.id.content_frame, fragment)
				.commit();

		mDrawerList.setItemChecked(possition, true);
		setTitle(dataList.get(possition).getItemName()); 
		mDrawerLayout.closeDrawer(mDrawerList);
    }

	
	@Override
	public void setTitle(CharSequence title) {
		mTitle = title.toString().replace("- ", "");
		getActionBar().setTitle(mTitle);
	}

	@Override
	protected void onPostCreate(Bundle savedInstanceState) {
		super.onPostCreate(savedInstanceState);
		mDrawerToggle.syncState();
	}

	@Override
	public void onConfigurationChanged(Configuration newConfig) {
		super.onConfigurationChanged(newConfig);
		mDrawerToggle.onConfigurationChanged(newConfig);
	}

	@Override
	public boolean onOptionsItemSelected(MenuItem item) {
		if (mDrawerToggle.onOptionsItemSelected(item)) {
			return true;
		}

		return false;
	}

	private class DrawerItemClickListener implements
			ListView.OnItemClickListener {
		@Override
		public void onItemClick(AdapterView<?> parent, View view, int position,
				long id) {
			SelectItem(position);
        }
	}

	public void oeffneDrawer(View view) {
		mDrawerLayout.openDrawer(mDrawerList);
	}

	@Override
	public void onBackPressed() {
		SelectItem(0);
		}

}

public class Training extends Fragment {

	public Training(byte kapitelnummer) {
		this.kapitelNummer = kapitelnummer;
	}

	byte kapitelNummer;

	Bildschirmgroesse bildschirmgroesse = new Bildschirmgroesse();
	int groesse = (int) bildschirmgroesse.getZoll();

	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
			Bundle savedInstanceState) {

		View view = inflater.inflate(R.layout.u_uebungen, container, false);


		return view;

	}

	
}