Try a full rebuild / clean. It’s possible that the a previous build has been abnormally aborted, leaving the ViewController.o file corrupted or zero-size.
To clean: In Xcode “Product” menu, select “Clean” (shortcut: Cmd+Shift+K)
To rebuild: In Xcode “Product” menu, select “Build” (shortcut: Cmd+B)
If it doesn’t work: In Xcode “Product” menu, hold down Option key, and select “Clean Build Folder”
reference from HERE.