WatchKit error - unable to find interface controller class '' to instantiate
I solved this issue by:
1) deleting trouble-making InterfaceController file and creating it again in the folder for WatchKit Extension in the project (not just in a group in Project navigator). By default xcode gives you Watch App folder - you have to change it.
2) setting class in Interface Builder so that module was set to ProjectName_WatchKit_Extension
If module is not set - xcode won't be able to find your class
For anyone else who has this. The class must be set to the correct target membership ie yourApp WatchKit Extension rather than yourApp WatchKit App when creating your class. It can also be edited in Target membership in interface builder.
I was having same issues after the recent 8.2 release. I had to go to each one of my InterfaceControllers and adjust its custom class module to whatever was shown as an option or "none".