Not Uploading An Asset To A Custom DAM Upload Field In Dialogs Causes An Error

Steps To Reproduce

• Add an upload field for assets to a dialog (can be a content app dialog or a page dialog - I've attached a sample field)
• Open this dialog
• Try to save the dialog without making any changes.

Expected Result
The dialog closes without any issues

Actual Result
Magnolia will throw the error attached.

error.txt error.rtf

Other Notes
Tested on most recent Magnolia version.
Happens on page dialogs and content app dialogs.