Hello Ivan Kopcanski,
If you are editing the control template, you should set the visibility of all items you are not willing to use to Collapsed. Otherwise, the RadWindow will fail since it requires it's buttons in the code behind. This should resolve your issue, however, you can still use the Popup control to show anything you want. This is the simplest control you can get that will help you solve your issues. If you have any other questions, we look forward to receiving your feedback.
the Telerik team