Fix crash when both FlutterFragmentActivity and FlutterFragment are destroyed and recreated (#25851)

Author: xster

shell/platform/android/io/flutter/embedding/android/FlutterFragmentActivity.java

@@ -390,6 +390,12 @@ FlutterFragment retrieveExistingFlutterFragmentIfPossible() {
    * FlutterFragment} is created and added.
    */
   private void ensureFlutterFragmentCreated() {
+    if (flutterFragment == null) {
+      // If both activity and fragment have been destroyed, the activity restore may have
+      // already recreated a new instance of the fragment again via the FragmentActivity.onCreate
+      // and the FragmentManager.
+      flutterFragment = retrieveExistingFlutterFragmentIfPossible();
+    }
     if (flutterFragment == null) {
       // No FlutterFragment exists yet. This must be the initial Activity creation. We will create
       // and add a new FlutterFragment to this Activity.
@@ -602,7 +608,7 @@ public FlutterEngine provideFlutterEngine(@NonNull Context context) {
    */
   @Override
   public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
-    if (flutterFragment.isFlutterEngineInjected()) {
+    if (flutterFragment != null && flutterFragment.isFlutterEngineInjected()) {
       // If the FlutterEngine was explicitly built and injected into this FlutterActivity, the
       // builder should explicitly decide whether to automatically register plugins via the
       // FlutterEngine's construction parameter or via the AndroidManifest metadata.

shell/platform/android/test/io/flutter/embedding/android/FlutterFragmentActivityTest.java

@@ -199,6 +199,26 @@ public void itRetrievesExistingFlutterFragmentWhenRecreated() {
     assertEquals(0, activity.numberOfEnginesCreated);
   }
 
+  @Test
+  public void itHandlesNewFragmentRecreationDuringRestoreWhenActivityIsRecreated() {
+    FlutterFragmentActivityWithProvidedEngine activity =
+        spy(Robolectric.buildActivity(FlutterFragmentActivityWithProvidedEngine.class).get());
+
+    FlutterFragment fragment = mock(FlutterFragment.class);
+    // Similar to the above case, except here, it's not just the activity that was destroyed and
+    // could have its fragment restored in the fragment manager. Here, both activity and fragment
+    // are destroyed. And the fragment manager recreated the fragment on activity recreate.
+    when(activity.retrieveExistingFlutterFragmentIfPossible()).thenReturn(null, fragment);
+
+    FlutterEngine engine = mock(FlutterEngine.class);
+    when(fragment.getFlutterEngine()).thenReturn(engine);
+
+    activity.onCreate(null);
+    // The framework would have recreated a new fragment but the fragment activity wouldn't have
+    // created a new one again.
+    assertEquals(0, activity.numberOfEnginesCreated);
+  }
+
   static class FlutterFragmentActivityWithProvidedEngine extends FlutterFragmentActivity {
     int numberOfEnginesCreated = 0;