Xamarin.Mac - NSInternalInconsistencyException

I'm suddenly getting this exception running 2 of my Xamarin.Mac .NET 4.5 applications. The UI's never get a chance to even show up. There're not new projects and I'm not sure if I ever ran them successfully after the XS update to 6.0.1 (build 9). Could there be a compatibility issue?

Exception Type: EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x0000000000000002, 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY

Application Specific Information:
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Invalid parameter not satisfying: nibNameMap != nil'

Application Specific Backtrace 1:
0 CoreFoundation 0x93ab6a69 __raiseError + 201
1 libobjc.A.dylib 0x9e79cfd1 objc_exception_throw + 276
2 CoreFoundation 0x93a4b2da +[NSException raise:format:arguments:] + 138

Does anybody have an idea what the problem is and how to fix it?

Thanks