With reference to this
forum thread: This behavior is expected - if you are creating the RadWindow object on the
client. When there is a postback, the page gets reloaded which means
that all dynamically created objects on that page, including RadWindow
will be destroyed. If you want to preserve them, you should use the
VisibleOnPageLoad property of the window and set it to true when you
want to show RadWindow on the page.