This a common error when using a Mx5 widget in a Mendix 6 project. Yesterday I tried to convert a Mx5 widget to Mx6 and I got that error too. Unfortunately the error doesn't point to the exact widget that is giving the error.
Unfortunately the only solution I can give you is to eliminate the widgets one-by-one (starting by the last one you downloaded from the appstore) to find the widget that is giving this error.
If you can post here which widget it is that is giving youthe error maybe somebody on the forum has a mx6 version of this widget.