Why Fragment crashes on .hide() with .setCustomAnimations()?

问题

xml:

    <RelativeLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent">



     <fragment
                android:id="@+id/the_fragment"
                android:name="com.example.MyFragment"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_alignParentBottom="true"
                tools:layout="@layout/fragment_my"/>

      <Button 
                android:id = "@+id/button"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Show/Hide"/>

    </RelativeLayout>

Code:

    @Override
                public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
                     super.onViewCreated(view, savedInstanceState);
                     mMyFragment = (MyFragment) getChildFragmentManager()
                                        .findFragmentById(R.id.the_fragment);
                        ((Button)view.findViewById(R.id.button)).setOnClickListener(view->{
        if (...){
        //hide
        getChildFragmentManager().beginTransaction()
.setCustomAnimations(R.animator.slide_in_from_bottom, R.animator.slide_out_to_bottom)
                                        .hide(mMyFragment)
                                        .commit();

        }else{
        //show
        getChildFragmentManager().beginTransaction()
.setCustomAnimations(R.animator.slide_in_from_bottom, R.animator.slide_out_to_bottom)//in this line the app crash
                                        .show(mMyFragment)
                                        .commit();
        }
        });

                                }

The error occurs:

Process: by.gramophone.develop, PID: 21508
    java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.ViewGroup.startViewTransition(android.view.View)' on a null object reference
        at android.support.v4.app.FragmentManagerImpl.completeShowHideFragment(FragmentManager.java:1700)
        at android.support.v4.app.FragmentManagerImpl.moveFragmentToExpectedState(FragmentManager.java:1797)
        at android.support.v4.app.BackStackRecord.executeOps(BackStackRecord.java:792)
        at android.support.v4.app.FragmentManagerImpl.executeOps(FragmentManager.java:2596)
        at android.support.v4.app.FragmentManagerImpl.executeOpsTogether(FragmentManager.java:2383)
        at android.support.v4.app.FragmentManagerImpl.removeRedundantOperationsAndExecute(FragmentManager.java:2338)
        at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:2245)
        at android.support.v4.app.FragmentManagerImpl$1.run(FragmentManager.java:703)
        at android.os.Handler.handleCallback(Handler.java:739)
        at android.os.Handler.dispatchMessage(Handler.java:95)
        at android.os.Looper.loop(Looper.java:148)
        at android.app.ActivityThread.main(ActivityThread.java:5417)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

UPDATE: But if I remove .setCustomAnimations(R.animator.slide_in_from_bottom, R.animator.slide_out_to_bottom) the error doesn't occurs

回答1:

As it is said on developer.android.com "When you add a fragment to an activity layout by defining the fragment in the layout XML file, you cannot remove the fragment at runtime. If you plan to swap your fragments in and out during user interaction, you must add the fragment to the activity when the activity first starts..." link

Try adding the fragment dynamically. how to

来源：https://stackoverflow.com/questions/51513173/why-fragment-crashes-on-hide-with-setcustomanimations