This happens only when editing a custom form? Did you save the form contents as a preset?
I think the problem is that the form contents preset doesn't exist any more.
presets are stored in the users configuration folder under configuration/shared/<product>
form content themes are stored under configuration/shared/FormBuilder/FormDefinitions/ContentStructures
The Javascript message, just means that there is code to initialize a spry element that no longer exists. that spry initialization line can be deleted from the page.