Got a crash in, of all things, System.Windows.Controls.Primitives.Popup.OnWindowResize(Object sender, AutoResizedEventArgs e). A NullReferenceException, to be precise. Ages later, it turns out that this error was actually caused by a binding operation on one of the controls in the popup failing because it was trying to instantiate itself as TwoWay with a read-only source property. Somehow, the binding error managed to turn into the NullReferenceException in the layout pass... ... Read more