I wonder what you’re really doing - because this is impossible to do under regular circumstances. In order to remove a default (which is set automatically by DEFAULT), you’d need to nullify the var and orphan the widget - that would cause the errors you see. Don’t do any of that.
If this is the button:
Do not assign anything here.
I’ll tell you one thing. I doubt anyone has ever seen an issue like this. I wonder if you added a variable manually at first, and then flagged the actual widget as variable and the names got mixed up…