Also make sure you do not bundle the widgets https://docs.mendix.com/refguide6/project-settings#bundle-widgets-when-running-locally So if one widget is broken it will not break all others.
Please note during upgrades I have seen 1 widget impacting others. For the testing purpose would recommend to remove/adding widgets one-by-one.
Is this an appstore widget? If so make sure to contact the developer responsible. Many widgets are still not compatible with Mx7+