React Native Share Extension won't build

I'm trying to add a Share Extension to a React Native app and when I try to build the app I get this linking error:

Undefined symbols for architecture x86_64:
  "_RCTRegisterModule", referenced from:
      _initialize_ShareManager in ShareManager-75b70a88150f3f544e5598c2fcbaaa49e4e22b3f47bce8d9a721bf31763b426d.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This is the offending code (when I comment this out the rest of the app builds fine):

@interface RCT_EXTERN_MODULE(ShareManager, NSObject)

RCT_EXTERN_METHOD(getURL: (RCTResponseSenderBlock)callback)

@end

Has anyone got any insight into how I can fix this? I've done a lot of googling and tried a hundred different permutations of linking binaries, different linker flags etc. But so far I've got nowhere.

Thanks for any help!