Unknown class FirstViewController in Interface Builder file

Go to project , targets , build phases , compile Sources and add the unknown class ..done.

It looks like you deleted FirstViewController.h & .m from project, but forgot to change view controller's class in nib file.

Right click on Storyboard --> Open As --> Source Code

Find your unknown class name in the storyboard source file. It may be because you have accidentally assigned a custom class name for one of your UIView subclass'.