I'm receiving the following exception when pinning a pane:
Error: Unhandled Error in Silverlight Application
Message: Failed to assign to property 'System.Windows.Controls.Primitives.Popup.Child'.
This only occurs after I've loaded RadDocking using LoadLayout from Xml that contains a RadPane that is unpinned.
So to reiterate, you need to load RadDocking with a layout that contains a pane that is unpinned. When you attempt to pin that pane the exception is thrown.
Can you replicate this your end?
I am using build Silverlight_4_2010_1_0430.