StoryBoard Assistant Editor stopped showing associated file
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:
- Close the project you are working on with.
- 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. - 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.