Check for a null codec in MultiFrameImageStreamCompleter after calling _emitFrame (#165009)

_emitFrame invokes image listeners. A listener callback could remove all
of the MultiFrameImageStreamCompleter's listeners, resulting in a
_maybeDispose call that destroys the codec.

Fixes flutter/flutter#164944
Fixes flutter/flutter#164537

Author: jason-simmons

packages/flutter/lib/src/painting/image_stream.dart

@@ -1047,6 +1047,10 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
       _frameDuration = _nextFrame!.duration;
       _nextFrame!.image.dispose();
       _nextFrame = null;
+      if (_codec == null) {
+        // codec was disposed during _emitFrame
+        return;
+      }
       final int completedCycles = _framesEmitted ~/ _codec!.frameCount;
       if (_codec!.repetitionCount == -1 || completedCycles <= _codec!.repetitionCount) {
         _decodeNextFrameAndSchedule();
@@ -1108,7 +1112,7 @@ class MultiFrameImageStreamCompleter extends ImageStreamCompleter {
       _nextFrame!.image.dispose();
       _nextFrame = null;
 
-      _codec!.dispose();
+      _codec?.dispose();
       _codec = null;
       return;
     }

packages/flutter/test/painting/image_stream_test.dart

@@ -1076,4 +1076,37 @@ void main() {
       areCreateAndDispose,
     );
   });
+
+  testWidgets('MultiFrameImageStreamCompleter image callback can remove listener', (
+    WidgetTester tester,
+  ) async {
+    final Completer<Codec> completer = Completer<Codec>();
+    final MockCodec mockCodec = MockCodec();
+    mockCodec.frameCount = 1;
+    final ImageStreamCompleter imageStream = MultiFrameImageStreamCompleter(
+      codec: completer.future,
+      scale: 1.0,
+    );
+
+    completer.complete(mockCodec);
+    await tester.idle();
+    expect(mockCodec.numFramesAsked, 0);
+
+    late ImageStreamListener streamListener;
+
+    void listener(ImageInfo image, bool synchronousCall) {
+      addTearDown(image.dispose);
+      imageStream.removeListener(streamListener);
+    }
+
+    streamListener = ImageStreamListener(listener);
+    imageStream.addListener(streamListener);
+    await tester.idle();
+    expect(mockCodec.numFramesAsked, 1);
+
+    final FrameInfo frame = FakeFrameInfo(const Duration(milliseconds: 200), image20x10);
+    mockCodec.completeNextFrame(frame);
+    await tester.idle();
+    expect(mockCodec.disposed, true);
+  });
 }