It looks like you may have found another bug. That error seems to be associated with clicking the icon to set a value for an area without an area selected in the tree above.
If you select an area from the tree first the error shouldn't occur. We'll make an adjustment to either disable that button when no tree item is selected or make sure there is always something selected in the tree.
You can create a design in form builder and save it and then use that design in the Data Assist wizard.