SyForms in Uipath error message after click submit button

Hi
I’m trying out SyForms in UiPath for the first time. I have follow tutorials, but I still got an error message (see picture). Sometime when I click on the button “Ferdig”, I got an error message, other times nothing happen.

In the design panel the “IsClosingForm” as True (see picture).

SyForms_UiPath2