From the information that you provided, I conclude that the code that you posted here is in the RadWindow's content page. If this is so, your logic will not work and the error is expected - this code should be executed in the main page where the tabstrip is.
Note that the content and the parent page are different, separate pages. If you want to use tabstrip's API to create a new tab, you should create a function in the main page that does that and then call this function from within the RadWindow's content page by using GetRadWindow().BrowserWindow.createTab();
where createTab is the name of the function on the parent page. More information on how to call functions in RadWindows is available here
the Telerik team