TODO:
You have a Telerik RadGrid and when you click on the Add button, you get the " Sys.ArgumentException: Value must not be null for Controls and Behaviors. Parameter name: element" error.
SOLUTION:
In my case I had the following structure:
Panel 1
FormGroup (a control we built)
Panel 2
RadGrid 1
I had AjaxSettings in my code behind for "Panel 1" and "FormGroup". Having AjaxSettings for the nested controls caused my issue. I removed the AjaxSetting for "FormGroup" and the issue went away
NOTES:
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