After I update to Xcode 10, The storyboard is acting very weird. Sometimes it's not showing anything for Identity, Attribute, Size, and Connection Inspector and even I clean the project also restart my Xcode and it's still the same. Here is an Image example:
How to solve this problem?
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.