M3:21 Don't see file uploader in testUploadMash during Preview


Hello there.
Am doing the mashup’s preview. Unfortunately can’t see fileupload widget.
Here are messages from the preview log:
16:49:10 INFO - In-page logger initialized.
16:49:10 INFO - ThingWorx Mashup Runtime v7.4.16-b153
16:49:11 ERROR - An error occurred while calling fileupload::renderHtml, Id = “fileupload-11”.
Exception: Cannot read property ‘1’ of null