On Mac, PyFerret seems enable to use an other window than window 1. SET WINDOW/NEW or SET WINDOW 2 crash :
objc: +[__NSPlaceholderDate initialize] may have been in progress in another thread when fork() was called.
objc: +[__NSPlaceholderDate initialize] may have been in progress in another thread when fork() was called. We cannot safely call it or ignore it in the fork() child process. Crashing instead. Set a breakpoint on objc_initializeAfterForkError to debug.
Tested with PyFerret 7.4 on Mac OS X Sierra, with python 3.7 and PyQt from HomeBrew
I summarized two methods of installation in this thread:
One of them shows the problem you see, but the other doesn't.
Specifically, on one of my Mac computers, I used the first method: you install the binary PyFerret package (.tar.gz). That gives the same error as yours.
On the other Mac computer of mine, I switched to the second method: you install miniconda and install PyFerret through conda. "Set window/new" works on this one.
Hope this helps,