We have a fix for the error you came across and it should ship in an upcoming release.
Also, I've seen this error when there are formulas consuming the Updates property of a Gallery control. I have seen users try to use the Gallery.Updates property as the rule for a Form control. This isn't going to achieve what you want. If you are using the form control in New mode (inserting new records instead of editing existing ones), then it is not required to have a rule for the Item property. Gallery.Updates also doesn't provide the form a record that would be editable in the form's data source.