Remove FlutterUndoManagerPlugin handlers from undo manager on dealloc (flutter/engine#53553)

When `FlutterUndoManagerPlugin` deallocated, it is supposed to deregister itself from the `NSUndoManager` stack.
However, during dealloc `_undoManagerDelegate.undoManager` is nil (`_undoManagerDelegate` = the engine, and `undoManager` was its view controller's `undoManager`, which was already gone).

Since `_undoManagerDelegate.undoManager` is nil, it doesn't actually call `-[NSUndoManager removeAllActionsWithTarget]`. In the add-to-app scenario, after the view controller pops back to the native view, and "undo" is invoked, the undo action for the `FlutterUndoManagerPlugin` is handled, but it already dealloced so crash.

Fixes https://github.com/flutter/flutter/issues/150408

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterUndoManagerPlugin.mm

@@ -13,6 +13,10 @@ static NSString* const kCanRedo = @"canRedo";
 
 @interface FlutterUndoManagerPlugin ()
 @property(nonatomic, weak, readonly) id<FlutterUndoManagerDelegate> undoManagerDelegate;
+
+// When the delegate is `FlutterEngine` this will be the `FlutterViewController`'s undo manager.
+// Strongly retain to ensure this target's actions are completely removed during dealloc.
+@property(nonatomic) NSUndoManager* undoManager;
 @end
 
 @implementation FlutterUndoManagerPlugin
@@ -28,13 +32,14 @@ static NSString* const kCanRedo = @"canRedo";
 }
 
 - (void)dealloc {
-  [_undoManagerDelegate.undoManager removeAllActionsWithTarget:self];
+  [_undoManager removeAllActionsWithTarget:self];
 }
 
 - (void)handleMethodCall:(FlutterMethodCall*)call result:(FlutterResult)result {
   NSString* method = call.method;
   id args = call.arguments;
   if ([method isEqualToString:kSetUndoStateMethod]) {
+    self.undoManager = self.undoManagerDelegate.undoManager;
     [self setUndoState:args];
     result(nil);
   } else {
@@ -43,11 +48,11 @@ static NSString* const kCanRedo = @"canRedo";
 }
 
 - (void)resetUndoManager {
-  [self.undoManagerDelegate.undoManager removeAllActionsWithTarget:self];
+  [self.undoManager removeAllActionsWithTarget:self];
 }
 
 - (void)registerUndoWithDirection:(FlutterUndoRedoDirection)direction {
-  NSUndoManager* undoManager = self.undoManagerDelegate.undoManager;
+  NSUndoManager* undoManager = self.undoManager;
   [undoManager beginUndoGrouping];
   [undoManager registerUndoWithTarget:self
                               handler:^(FlutterUndoManagerPlugin* target) {
@@ -64,7 +69,7 @@ static NSString* const kCanRedo = @"canRedo";
 }
 
 - (void)registerRedo {
-  NSUndoManager* undoManager = self.undoManagerDelegate.undoManager;
+  NSUndoManager* undoManager = self.undoManager;
   [undoManager beginUndoGrouping];
   [undoManager registerUndoWithTarget:self
                               handler:^(id target) {
@@ -76,7 +81,7 @@ static NSString* const kCanRedo = @"canRedo";
 }
 
 - (void)setUndoState:(NSDictionary*)dictionary {
-  NSUndoManager* undoManager = self.undoManagerDelegate.undoManager;
+  NSUndoManager* undoManager = self.undoManager;
   BOOL groupsByEvent = undoManager.groupsByEvent;
   undoManager.groupsByEvent = NO;
   BOOL canUndo = [dictionary[kCanUndo] boolValue];

engine/src/flutter/shell/platform/darwin/ios/framework/Source/FlutterUndoManagerPluginTest.mm

@@ -27,6 +27,7 @@ FLUTTER_ASSERT_ARC
 
 @property(readonly) NSUInteger undoCount;
 @property(readonly) NSUInteger redoCount;
+@property(nonatomic, nullable) NSUndoManager* undoManager;
 
 - (instancetype)initWithUndoManager:(NSUndoManager*)undoManager
                 activeTextInputView:(TextInputViewTest*)activeTextInputView;
@@ -164,4 +165,38 @@ FLUTTER_ASSERT_ARC
   OCMVerify(never(), [self.activeTextInputView inputDelegate]);
 }
 
+- (void)testDeallocRemovesAllUndoManagerActions {
+  __weak FlutterUndoManagerPlugin* weakUndoManagerPlugin;
+  // Use a real undo manager.
+  NSUndoManager* undoManager = [[NSUndoManager alloc] init];
+  @autoreleasepool {
+    id activeTextInputView = OCMClassMock([TextInputViewTest class]);
+
+    FakeFlutterUndoManagerDelegate* undoManagerDelegate =
+        [[FakeFlutterUndoManagerDelegate alloc] initWithUndoManager:undoManager
+                                                activeTextInputView:activeTextInputView];
+
+    FlutterUndoManagerPlugin* undoManagerPlugin =
+        [[FlutterUndoManagerPlugin alloc] initWithDelegate:undoManagerDelegate];
+    weakUndoManagerPlugin = undoManagerPlugin;
+
+    FlutterMethodCall* setUndoStateCall =
+        [FlutterMethodCall methodCallWithMethodName:@"UndoManager.setUndoState"
+                                          arguments:@{@"canUndo" : @YES, @"canRedo" : @YES}];
+    [undoManagerPlugin handleMethodCall:setUndoStateCall
+                                 result:^(id _Nullable result){
+                                 }];
+    XCTAssertTrue(undoManager.canUndo);
+    XCTAssertTrue(undoManager.canRedo);
+    // Fake out the undoManager being nil, which happens when the FlutterViewController deallocs and
+    // the undo manager can't be fetched from the FlutterEngine delegate.
+    undoManagerDelegate.undoManager = nil;
+  }
+  XCTAssertNil(weakUndoManagerPlugin);
+  // Regression test for https://github.com/flutter/flutter/issues/150408.
+  // Undo manager undo and redo stack should be empty after the plugin deallocs.
+  XCTAssertFalse(undoManager.canUndo);
+  XCTAssertFalse(undoManager.canRedo);
+}
+
 @end