You say that your Grid reloads after uploading the PDF file. Is there a reason why you need the Grid to reload again? If I understand correctly, there are no changes to the Grid itself when a PDF upload happens.
Also, about the DropDownList error, can you provide more details on where you have a DropDownList in the Grid - do you have any custom DropDownLists that you added as templates?
Can you show us the logic that reloads the Grid on PDF upload and any initialization code for custom DropDownLists in your Grid?
In the meantime, make sure that you don't have any custom code that might be running while the Grid is reloading, or before its column menu is initialized.
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