I just had the same problem and tried the above solutions. What worked for me was a clean of the project.
In the menu bar go to…
Product –> Clean
When that is complete hold down the option key and again in the menu bar go to…
Product –> Clean Build Folder
I’m not sure if the clean of the build folder was necessary. It’s just become habit for me to do both. I have to do this more than I would like in Xcode.