When trying to Add Joins or Remove Joins, the following error message is generated:
System.NullReferenceException: Object reference not set to an instance of an object.
at Haestad.ArcGISPro.Library.ArcGISUtil.HasJoins(MapView mapView)
The issue is related to an issue geodatabase file To resolve the issue, you can try to move the saved all the .APRX and associated WaterGEMS or SewerGEMS files a new project name in a new folder.
If this doesn't help, the issue may be related to a specific layer that needs to be added to the map from the personal geodatabase file. Contact Bentley Technical Support to send the model files.
Overview of WaterGEMS and SewerGEMS for ArcGIS Pro