Yet another possible cause:
In my case, I had a xib and was calling Bundle.main.loadNibNamed(... at which point it was crashing. All my IBOutlets and IBActions seemed fine. In the end I noticed that the nib was actually loading, but then when I tried to access the first (and only) view (nib.first), that was not working. In fact nib.count was returning a very large integer (max int?), instead of the expected 1. I tried re-adding the view, but still no luck.
In the end, I deleted the whole xib file and started again (but I kept some of the views in my clipboard so I can paste them instead of recreating them again).
This worked!
