Xcode 10: Storyboard not showing Identity, Attribute... Inspector

Click on Version editor in the right uppermost corner.

Now again click on Assistant editor

Just select your storyboard tab, duplicate it (CMD+T) and close the first one. The new storyboard tab should work properly now.

Alex Jackson's answer did not work for me. Try clicking on the 'Show Assistant Editor' button in the top right. That worked for me.

Try to go to File -> Workspace Settings -> Go to derived data location -> delete your project inside it! then clean and build your project