why is Xamarin giving error with button Clicked?

This is an old question, but I ran into a similar issue. There was no good explanation why it would give the error. It would even happen on an empty handler that the IDE generated itself. I tried cleaning the solution, rebuilding, etc.

I found that there were some stuck build tasks going still. They appeared in the bottom bar that normally shows "Ready". For me, the solution was restarting VS. Once it started back up, and build worked fine.