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b6afc16a96
There were 2 possible scenarios in which _handleAppFrame is added more than once as a frame callback. When this happens it is possible that the second invocation will try to access _nextFrame.image when _nextFrame is null and crash. The 2 scenarios are: Scenario 1 A GIF frame is decoded and a Flutter frame is executed before it's time to show the next GIF frame. The timer that's waiting for enough time to elapse is invoked, and schedules a callback for the next Flutter frame(here). Before the next Flutter frame is executed, MultiFrameImageStreamCompleter#removeListener is called followed by ``MultiFrameImageStreamCompleter#addListenerthat is invoking_decodeNextFrameAndSchedule` which is adding `_handleAppFrame` again as a next frame callback. Scenario 2 removeListener and addListener are called multiple times in succession, every call to addListener can result in another registration of _handleAppFrame to the next Flutter frame callbacks list. This patch fixes the issue by guarding against a second registration of _handleAppFrame.