We do not support the DNN environment and if you have issues with it that do not manifest in a "regular" aspx page I advise reaching out to their community in their specific forums.
Another possible reason for a problem would be that the script is, indeed, not registered. Since this is a standard .NET method, the issue would be probably in the DNN site and what I can suggest at this point is that you try the following syntax:
You can also try registering a simple alert() in the Sys.Application.Load event to see if that fires. If it does not - then the problem is with the way DNN works and registers scripts. In case the $find() call returns null, you can try adding a small timeout in the code to give the browser a bit more time to create the client-side objects.
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