Problem with WindowShouldClose

I'm handling WindowShouldClose event in my application like this
public override void ViewWillAppear() { base.ViewWillAppear(); View.Window.WindowShouldClose += Window_WindowShouldClose; }
It used to work until I've moved to new machine with new installation. I guess it is caused by newer Mono or Xamarin, now it throws exception:
Event registration is overwriting existing delegate. Either just use events or your own delegate: TestApp.WindowController AppKit.NSWindow+_NSWindowDelegate
I don't have any class inheriting from NSWindow. How can I solve this? Thank you.