片段动画的例外情况

Question

我在app中有两个片段。切换它们的代码：

public class MainActivity extends FragmentActivity{

    Fragment1 fragment1;
    Fragment2 fragment2;


    FragmentManager fragmentManager;
    FragmentTransaction fragmentTransaction;


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.some_layout);

        fragment1 = new Fragment1();
        fragment2 = new Fragment2();

        fragmentManager = getSupportFragmentManager();

        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.setCustomAnimations(R.anim.fragment_vert_add, R.anim.fragment_vert_remove);
        fragmentTransaction.add(R.id.linear_layout_id, fragment1);
        fragmentTransaction.commit();
    }

    public void changeFragment() {

        fragmentTransaction = fragmentManager.beginTransaction();
        fragmentTransaction.setCustomAnimations(R.anim.fragment_vert_add, R.anim.fragment_vert_remove);

        if(fragment1.isAdded())
            fragmentTransaction.replace(R.id.linear_layout_id, fragment2);
        else
            fragmentTransaction.replace(R.id.linear_layout_id, fragment1);
        fragmentTransaction.commit();
    }

}

在动画期间触摸屏幕时：

E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.IllegalStateException: No activity 
     at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1123)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1118)
            at android.support.v4.app.FragmentManagerImpl.dispatchActivityCreated(FragmentManager.java:1927)
            at android.support.v4.app.Fragment.performActivityCreated(Fragment.java:1800)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:977)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1136)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:739)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1499)
            at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:456)
            at android.os.Handler.handleCallback(Handler.java:605)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4424)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
            at dalvik.system.NativeStart.main(Native Method)