Usually your code should work properly even after postback.
Is it possible that the ID of the window gets changed after postback or the window object not created due to an error?
Here is how to inspect what ID is generated before and after postback:
Right click over the window title icon, choose Inspect from the browser content menu and go to the parent div wrapper of the window to see the ID, which should be something similar to:
at least this is the div wrapper ID in the Overview demo: https://demos.telerik.com/aspnet-ajax/window/examples/overview/defaultcs.aspx
Delete the beginning RadWindowWrapper_ string to get the real ID of the window, which will allow you to get reference to it via the $find("ctl00_ContentPlaceholder1_RadWindow1") method.
Do the same inspection after the postback to see whether the ID has been changed and a different one assigned to the window.
For your convenience I created a video demonstration.
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