BadTokenException ActivityGroup for TabActivity using

I need to implement nested actions in TabHost in my application. I am using ActivityGroup class like this:

  public class CustomActivityGroup extends ActivityGroup {

View rootView;


public static CustomActivityGroup group;


private ArrayList<View> history;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    this.history = new ArrayList<View>(); group = this;



      View view = getLocalActivityManager().startActivity("LoginActivity", new Intent(this,RegisterActivity.class) .addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP))
      .getDecorView();
      replaceView(view);


}

@Override
protected void onResume() {

    super.onResume();
    this.history = new ArrayList<View>();
    group = this;

}

public void replaceView(View v) {

    if (history.size() == 0) {
        if (rootView != null) {
            history.add(rootView);
            rootView = null;
        }
    }
    history.add(v);  
    setContentView(v);
}

public void back() {
    try {
        if (history.size() > 0) {
        history.remove(history.size() - 1);
        if (history.size() > 0) {
            setContentView(history.get(history.size() - 1));
        } else {
            finish();
        }
    } else {
        finish();
    }
    } catch (Exception ex) {
    }
}

public int getHistorySize() {
    return history.size();
}

@Override
public void onBackPressed() {
    try {
        CustomActivityGroup.group.back();
    } catch (Exception ex) {

    }
    return;
}
}

      

        
        
        
      

    

and in my TabActivity:

  intent = new Intent().setClass(this, CustomActivityGroup.class);

    spec = tabHost.newTabSpec("tab_2").setIndicator("Sign up",getResources().getDrawable(R.drawable.signup)).setContent(intent);
    tabHost.addTab(spec);

      

        
        
        
      

    

but when i click on the corresponding tab i get

 FATAL EXCEPTION: main
 java.lang.RuntimeException: Unable to start activity ComponentInfo{com.assignmentexpert/com.activitygroups.CustomActivityGroup}: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.assignmentexpert/com.assignmentexpert.RegisterActivity}: android.view.WindowManager$BadTokenException: Unable to add window -- token android.app.LocalActivityManager$LocalActivityRecord@405ceb00 is not valid; is your activity running?
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
    at android.app.ActivityThread.startActivityNow(ActivityThread.java:1491)
    at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)

    at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
    at android.widget.TabHost$IntentContentStrategy.getContentView(TabHost.java:676)
    at android.widget.TabHost.setCurrentTab(TabHost.java:348)
    at android.widget.TabHost$2.onTabSelectionChanged(TabHost.java:141)
    at android.widget.TabWidget$TabClickListener.onClick(TabWidget.java:456)
    at android.view.View.performClick(View.java:2485)
    at android.view.View$PerformClick.run(View.java:9080)
    at android.os.Handler.handleCallback(Handler.java:587)
    at android.os.Handler.dispatchMessage(Handler.java:92)
    at android.os.Looper.loop(Looper.java:123)
    at android.app.ActivityThread.main(ActivityThread.java:3687)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:507)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.assignmentexpert/com.assignmentexpert.RegisterActivity}: android.view.WindowManager$BadTokenException: Unable to add window -- token android.app.LocalActivityManager$LocalActivityRecord@405ceb00 is not valid; is your activity running?
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1651)
    at android.app.ActivityThread.startActivityNow(ActivityThread.java:1491)
    at android.app.LocalActivityManager.moveToState(LocalActivityManager.java:127)
    at android.app.LocalActivityManager.startActivity(LocalActivityManager.java:339)
    at com.activitygroups.CustomActivityGroup.onCreate(CustomActivityGroup.java:31)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1615)
    ... 18 more
 Caused by:  Unable to add window -- token android.app.LocalActivityManager$LocalActivityRecord@405ceb00 is not valid; is your activity running?
    at android.view.ViewRoot.setView(ViewRoot.java:532)
    at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:177)
    at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
    at android.view.Window$LocalWindowManager.addView(Window.java:424)
    at android.app.Dialog.show(Dialog.java:241)
    at com.asynctaskbase.AbstractTaskProgressDialogFragment.onCreateDialog(AbstractTaskProgressDialogFragment.java:68)
    at com.asynctaskbase.AbstractTaskProgressDialogFragment.onCreateDialog(AbstractTaskProgressDialogFragment.java:1)
    at android.support.v4.app.DialogFragment.getLayoutInflater(DialogFragment.java:295)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:871)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1083)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:635)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1431)
    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:523)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1129)
    at android.app.Activity.performStart(Activity.java:3791)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1624)
    ... 24 more

      

        
        
        
      

    

What am I doing wrong?