BTW, looks like this is a SP1 issue.

The WindowState property is a member of UXRibbonWindow class.
Could you please inform me with a step-by-step guide to modify the ClientUI_Ribbon_Application development sample that I can use to observe the reported problem?

Edited on February 15, 2012 11:55 PM
Reason: Update Latest Status

I have managed to be able to reproduce the reported problem in Intersoft ClientUI Ribbon Application (WPF) project template. It is submitted and filed under work item #1137.

I'll keep you informed with any news I heard from the team regarding work item #1137.

Edited on February 16, 2012 03:47 AM
Reason: Update Latest Status (Solved)

ClientUI development team has managed to fix the problem submitted under work item #1137. I have tested the pre-release hotfix on my local end and find that the "object reference not set to an instance of an object" exception is no longer persist when WindowState property is set to "Maximized".

Please feel free to let us know if you need to test the pre-release hotfix on your local end.