I added a UIView xib file using the root class of
Unknown Class MyView in Interface Builder file
OK, so I deleted all references to the class causing the problem but still nothing.
However, I also had a UIViewController subclass called
MyViewController. This wasn't being loaded in any IB file so I don't know how it was causing the problem.
I changed the name to
Build - Run - works.
I don't know how or why but it works now.