ArgumentNullException when Diagram is not hosted in first RadTabItem

[Christo]

As soon as you host a RadDiagram inside a RadTabItem which is not the first RadTabItem in the collection you get the following exception when you click on the settings pane icon:

System.ArgumentNullException was unhandled by user code
  Message=Value cannot be null.
Parameter name: source
  Source=System.Core
  ParamName=source
  StackTrace:
       at System.Linq.Enumerable.Select[TSource,TResult](IEnumerable`1 source, Func`2 selector)
       at Telerik.Windows.Controls.Diagrams.Extensions.SettingsPaneView.UpdateEditor(DependencyObject editor) in c:\TB\105\WPF_Scrum\Release_WPF\Sources\Development\Diagrams\Extensions\SettingsPane\SettingsPaneView.cs:line 321
       at Telerik.Windows.Controls.Diagrams.Extensions.SettingsPaneView.OnRegisterEditor(Object sender, RadRoutedEventArgs args) in c:\TB\105\WPF_Scrum\Release_WPF\Sources\Development\Diagrams\Extensions\SettingsPane\SettingsPaneView.cs:line 265
  InnerException: 

I sat for hours and hours to figure out the cause of this problem. 

It works fine when the RadDiagram get's hosted in the first TabItem but if you move it to any other TabItem you start getting problems with the settingspane.

Here is a few screenshots to demonstrate the problem:
http://screencast.com/t/mfWlB20nNP
http://screencast.com/t/ccXGMklHFe
Here is a video to demonstrate:
http://screencast.com/t/5khAEDE4 

[Zarko]

Hello Christo,
Thank you for the feedback! I was able to reproduce the issue and I've logged it in our PITS under the name "The SettingsPane throws an exception when in RadTab" and it'll be ready for tracking and voting tomorrow the latest.
I've updated your telerik account and if you have further questions please feel free to ask.

Greetings,
Zarko
the Telerik team