The popup that is being generated is created by XFile itself. Error handling around the Start method will not prevent it from being displayed. In order to properly supress that dialog, the SuppressPopupTransferError property must be set equal to the boolean value True. This property is part of both visual control objects, AXFFile and AXFFileDownload.
Ex:
AXFFile.SuppressPopupTransferError = true
or
AXFFileDownload.SuppressPopupTransferError = true
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