When you place a RadEditor in the ContentTemplate of a RadWindow the buttons no longer function. This holds true for any scenario in which you move the RadEditor in the DOM.
There is an easy fix that causes the RadEditor to reattach its handlers and start functioning again - this is done by calling its onParentNodeChanged() method when you move it, which, in this case, is the OnClientShow event of the RadWindow:
Check the following Telerik help documentation which explains more about your issue. RadEditor Does Not Work in RadWindow