In macOS High Sierra if getting this:

objc[55492]: +[__NSPlaceholderDate initialize] may have been in progress in another thread when fork() was called.
objc[55492]: +[__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.

Define:

export OBJC_DISABLE_INITIALIZE_FORK_SAFETY=YES

This will fix temporally fix the problem and allow program to continue executing, more about this issue here: http://sealiesoftware.com/blog/archive/2017/6/5/Objective-C_and_fork_in_macOS_1013.html