After a lot of search and reading posts, here's what fixed it for me. It the Targets>[your app]>General tab, I had not yet set my app icon name in the App Icons and Launch Screen section yet. It had placeholder text "App Icon" that I incorrectly read as "AppIcon", which is what mine is called. Once I added that the problem was fixed.

I have been trying to fix this - thanks for your post. I realized I had not yet typed in my app's icon name in Targets/[my app]/General App Icon section. the placeholder text in there was "App Icon" and I just assumed it was "AppIcon" which is the actual name of my app's icon set.