I cannot access the Admin / File Management after a DNN upgrade.

DNN was upgraded from DNN 7.1.2 to 7.2.1.

When I try to go to the Admin / File Management page, I now get an error:

A critical error has occurred.Only one instance of the DnnFileUpload can be contained by the page.

1. I went to Page Management and view the modules on the page. There is only one module, the Digital Asset Management module. I deleted that module from the page
2. I browsed back to File Management (and I could successfully view the page)
3. I re-added Digital Asset Management, but it gives me the same error again.