This is due to a coding error at the client-side, an incorrectly handled unhappy flow. During happy flow the clientside widget will have previously found the object, and the ‘in’ is able to make the comparison. During unhappy flow, no object is found and the error is thrown. Probably a “for x in y” where y is null.
If this is not your code/widget that is throwing the error, your first job is to find out which widget is creating this error. The errormessage gives you little to no information on this. Start removing widgets from your project-directory /widgets (or just rename for easy getting them back).
Since there is a good chance that you might end up needing to update a widget: Menu ‘App’=>’Tools’=>’Update widgets’.
Ok thanks I will start that process :) hopefully it will go well.