Fix _transitionEnd method to return early if _map is not defined

[JingWTG]

Our app encountered an error where this._map was undefined when the callback function in _transitionEnd() was executed. Since Util.requestAnimFrame() is an asynchronous operation, the map reference can become undefined by the time the callback runs (e.g., if the layer is removed from the map before the animation frame is processed).

[gavinr-maps]

Hi, thank you for the PR. Are you able to post a public replication case to demonstrate the issue you're fixing? https://jsbin.com/jexevoy/edit?html,output

[JingWTG]

Hi, thank you for the PR. Are you able to post a public replication case to demonstrate the issue you're fixing? https://jsbin.com/jexevoy/edit?html,output

Hi, I couldn’t reproduce it — the error came from our error reporting system.

[JingWTG]

Message
Cannot read properties of null (reading 'getZoom')