In my case none of the solutions worked yet. The bootflag solution with up-to-date kexts doesn't work (iTunes still crash) and I can't change my system definition, because my Dell U4919DW won't work. Only the MacPro supports the native resolution of my monitor over DisplayPort. So any other suggestions to fix the iTunes crashes under 10.14.5?
//Edit:
Okay, I found my solution. Like I said, I need the Mac Pro system definition for the support of my Dell monitor with the nativ resolution of 5120x1440. So I simply change the definition from MacPro6,1 to MacPro5,1. The monitor still works, iTunes runs like a charm.