When Assistant doesn't work in Xcode

I often have a problem in Xcode when I am working in the Storyboard, when I click on “Assistant” to open the related ViewController to add an Outlet or Action, and Assistant can’t find the related file.

The workaround to this is instead of clicking the “Assistant” button, option-click the corresponding ViewController file in the Project Navigator, which will open that file in the same way that “Assistant” would, so you can ctl-drag the Outlets and Actions.