Fix crash when FlutterFragmentActivity is recreated with an existing FlutterFragment

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

@@ -241,6 +241,10 @@ public Intent build(@NonNull Context context) {
   @Override
   protected void onCreate(@Nullable Bundle savedInstanceState) {
     switchLaunchThemeForNormalTheme();
+    // Get an existing fragment reference first before onCreate since onCreate would re-attach
+    // existing fragments. This would cause FlutterFragment to reference the host activity which
+    // should be aware of its child fragment.
+    flutterFragment = retrieveExistingFlutterFragmentIfPossible();
 
     super.onCreate(savedInstanceState);
 
@@ -366,21 +370,31 @@ private View createFragmentContainer() {
     return container;
   }
 
+  /**
+   * Retrieves the previously created {@link FlutterFragment} if possible.
+   *
+   * <p>If the activity is recreated, an existing {@link FlutterFragment} may already exist. Retain
+   * a reference to that {@link FlutterFragment} in the {@code #flutterFragment} field and avoid
+   * re-creating another {@link FlutterFragment}.
+   */
+  @VisibleForTesting
+  FlutterFragment retrieveExistingFlutterFragmentIfPossible() {
+    FragmentManager fragmentManager = getSupportFragmentManager();
+    return (FlutterFragment) fragmentManager.findFragmentByTag(TAG_FLUTTER_FRAGMENT);
+  }
+
   /**
    * Ensure that a {@link FlutterFragment} is attached to this {@code FlutterFragmentActivity}.
    *
    * <p>If no {@link FlutterFragment} exists in this {@code FlutterFragmentActivity}, then a {@link
-   * FlutterFragment} is created and added. If a {@link FlutterFragment} does exist in this {@code
-   * FlutterFragmentActivity}, then a reference to that {@link FlutterFragment} is retained in
-   * {@code #flutterFragment}.
+   * FlutterFragment} is created and added.
    */
   private void ensureFlutterFragmentCreated() {
-    FragmentManager fragmentManager = getSupportFragmentManager();
-    flutterFragment = (FlutterFragment) fragmentManager.findFragmentByTag(TAG_FLUTTER_FRAGMENT);
     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.
       flutterFragment = createFlutterFragment();
+      FragmentManager fragmentManager = getSupportFragmentManager();
       fragmentManager
           .beginTransaction()
           .add(FRAGMENT_CONTAINER_ID, flutterFragment, TAG_FLUTTER_FRAGMENT)

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

@@ -3,6 +3,7 @@
 import static io.flutter.embedding.android.FlutterActivityLaunchConfigs.HANDLE_DEEPLINKING_META_DATA_KEY;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.spy;
@@ -170,7 +171,37 @@ public void itAllowsRootLayoutOverride() {
     assertTrue(foundCustomView);
   }
 
+  @Test
+  public void itCreatesAValidFlutterFragment() {
+    FlutterFragmentActivityWithProvidedEngine activity =
+        Robolectric.buildActivity(FlutterFragmentActivityWithProvidedEngine.class).get();
+
+    // Creating the FlutterFragmentActivity will create and attach the FlutterFragment, causing
+    // a FlutterEngine to be created.
+    activity.onCreate(null);
+    assertNotNull(activity.getFlutterEngine());
+    assertEquals(1, activity.engineCreated);
+  }
+
+  @Test
+  public void itRetrievesExistingFlutterFragmentWhenRecreated() {
+    FlutterFragmentActivityWithProvidedEngine activity =
+        spy(Robolectric.buildActivity(FlutterFragmentActivityWithProvidedEngine.class).get());
+
+    FlutterFragment fragment = mock(FlutterFragment.class);
+    when(activity.retrieveExistingFlutterFragmentIfPossible()).thenReturn(fragment);
+
+    FlutterEngine engine = mock(FlutterEngine.class);
+    when(fragment.getFlutterEngine()).thenReturn(engine);
+
+    activity.onCreate(null);
+    assertEquals(engine, activity.getFlutterEngine());
+    assertEquals(0, activity.engineCreated);
+  }
+
   static class FlutterFragmentActivityWithProvidedEngine extends FlutterFragmentActivity {
+    int engineCreated = 0;
+
     @Override
     protected FlutterFragment createFlutterFragment() {
       return FlutterFragment.createDefault();
@@ -184,6 +215,7 @@ public FlutterEngine provideFlutterEngine(@NonNull Context context) {
       when(flutterJNI.isAttached()).thenReturn(true);
       when(flutterLoader.automaticallyRegisterPlugins()).thenReturn(true);
 
+      engineCreated++;
       return new FlutterEngine(context, flutterLoader, flutterJNI, new String[] {}, true);
     }
   }