Thank you for your sample project.
Initialy, having xaml in both UserControls - Container and Window - is not a good practise. The error is because it is trying to call the Loaded handler from code-behind of the inherited class, not from the parent in which code-behind the handler is handled. If you place the Loaded event handler in the code-behind of the inherited class the exception will disappear, but this is not recommended.
Could you please share with us what is your scenario - why do you need to inherit the RadWindow like this? In that way we will be able to assist you in the best manner.
If you have further inquiries please let us know.
the Telerik team
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