I have made multiple solutions with Ninitex Forms and Workflow for Office 365 and this is the first time I have run into this.
I originally ran into this error with a Personal Time Off request using a SharePoint list. I couldn't find the answer so I just deleted everything and started over. It all worked great for a few days, but today I started getting the error again. This makes it so no one can actually submit a request with the form I have created. The error seems to happen after I have made many design changes to the form.
I've attached a screen clipping of the error and a screen clipping of the form in the designer just incase that is helpful.
We were able to reproduce this issue in testing and our Dev team is currently investigating.
We should have an update soon from the Dev team.
This can be caused by column name, or another example:
When a default value in a disabled people picker control is equal to the empty string (""). This should only be possible if a token resolves to nothing. For example, Created By (Display Name) will resolve to nothing if the item is in New mode.
Leaving the default value field empty will pass a null default value, and so will not cause this error.
Best thing to do is to open a support case so we can review what elements are in play here. Please be sure to include the form export as well as the list export.
Figured out the issue, if anyone is interested, my issue was related to a people and group field.
----- that was the issue.
Solution for me:
People and group form field -- should be connected to -- people and group list column (not a text column)