# WPF: Subtle Binding Crash

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…

Just throwing that out there for anyone that might be seeing something similarly awful.

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