The library is looking for an /usr/local/emulator folder (but it was installed in another directory). First I checked whether the directory existed (it didn't), found where the installation directory actually was, then created a symbolic link to the actual folder location. Finally, check the requested directory exists, then everything worked.
i.e.
$ ls -la /usr/local/emulator
ls: /usr/local/emulator: No such file or directory
$ which emulator
/usr/local/bin/emulator
$ ls -la /usr/local/bin/emulator
lrwxr-xr-x 1 user admin 64 13 Aug 09:56 /usr/local/bin/emulator -> /usr/local/Caskroom/android-sdk/3859397,26.0.1/emulator/emulator
$ sudo ln -s /usr/local/Caskroom/android-sdk/3859397,26.0.1/emulator /usr/local/emulator
Password:
$ ls "/usr/local/bin/../emulator/qemu/darwin-x86_64/qemu-system-i386"
/usr/local/bin/../emulator/qemu/darwin-x86_64/qemu-system-i386