I'm getting the following error using UpdatePanelAnimationExtender: Unable to get property 'id' of undefined or null reference. This did not happen prior to v15.1.3. My guess is it has something todo with the fix in workitem
The exception is thrown when clicking a row in the GridView. I've attached a sample application showing the problem. The exception only occurs once per full page refresh, but the onUpdating and onUpdated script methods never run after that.
By commenting out the code in GridView1_RowDataBound and the hiddenColumn css class and use the "Select" link button on the GridView to select a row instead then no exception is thrown.