Xcode stores some user state information inside the Xcode project “file”, which is really a folder. That state information might have become corrupted. You can normally throw away everything inside your .xcodeproj folder except the project.pbxproj file. This might fix your problem.
Open up the folder containing your .xcodeproj file. Right-click or control-click the .xcodeproj file and choose “Show Package Contents”. Then throw away everything except the project.pbxproj file.
If you know what an Xcode workspace is, and you’re actually making use of it, you might not have a project.xcworkspace file to throw away, or you might not want to throw it away. But if you don’t know what an Xcode workspace is, then you’re not using it so you can just throw away project.xcworkspace. Xcode will recreate it automatically.