StoryBoard Assistant Editor stopped showing associated file

In same case this worked for me:

1. Right click on ViewController file(on left pane) and choose Delete -> Remove Reference.
2. Right click on folder where this ViewController file was and choose Add files to .. and add this ViewController file which your just deleted.
3. Profit.

Deleting the Derived Data didn't work for me nor force quitting Xcode and restarting it.

The only thing that worked was deleting both the class and the storyboard (only the reference to them) and adding them back to the project.

Hope to help someone.

Found the solution (at least worked for me), it's the same bug that causes you not to be able to create new outlets. You must delete the [DerivedData] folder:

1. Close the project you are working on with.
2. Delete the【DerivedData】folder of you project. (This folder may inside your project's folder, or inside ~/Library/Developer/XCode/DerivedData/(your project)/ ) or somewhere else that were setup by you.
3. Restart Xcode.

see here.

With XCode 11 onwards, you can option(alt) + click on the file that you want to view in assistant editor. Simple Fix.