From last two days, I am able to open the assistant editor and when I opened my other projects they were also having the same error.
Also, the new ones that I made, had the same error. Therefore, I am able to establish a connection between view controller and storyboard, and there was a lock icon on the side of the assistant editor that I tried to open.
When I tried to unlock the lock icon it showed:
The file is a remote resource. Try making a local copy.
I have deleted derived data also but didn't get any success.
Your Xcode IDE default setting change to false use the below step:
defaults write com.apple.dt.Xcode IDEIndexDisable -bool false
then press enterHappy Coding :)