Can't build app because of 2 paths for libswiftCore.dylib

Question

After upgradeing to Mojave my rubymotion (6.1) app doesn't compile. I get the following error:

Class _TtCs18__stdlib_AtomicInt is implemented in both /usr/lib/swift/libswiftCore.dylib (0x7fffa61010c8) and /Applications/Xcode.app/Contents/Frameworks/libswiftCore.dylib (0x1072be158).
One of the two will be used. Which one is undefined.
Class _TtCs19__EmptyArrayStorage is implemented in both /usr/lib/swift/libswiftCore.dylib (0x7fffa60f31e0) and /Applications/Xcode.app/Contents/Frameworks/libswiftCore.dylib (0x1072b0268).
One of the two will be used. Which one is undefined.
...
"This copy of libswiftCore.dylib requires an OS version prior to 10.14.4"

Answer 1

It was fixed by the Rubymotion maintainers. Running it again with an updated Rubymotion I got an error message which explained what to do.

I had to run:

sudo cp -r /usr/lib/swift/*.dylib /Applications/Xcode.app/Contents/Frameworks
sudo touch /Applications/Xcode.app/Contents/Frameworks/.swift-5-staged