This was driving me crazy for a long time. I found nothing here on Stackoverflow about this, and not by googling either, initially.
Luckily, after googling a couple of different ways, I finally found the simple solution here.
Snipped from that site:
Here’s how you can fix it in two steps without even having to restart
Xcode:
- Select another file (which isn’t an Interface Builder type) in the current tab with the issue.
- Open a new tab and select the Interface Builder file which was having the problem.
Shame on you, Apple!