Try to Fix: an internal error has occurred. editing functionality may be limited xcode 9.4

if a message or error appears like this, then try some tricks like below

// Clean drive data 

1. xcode

2. preference

3. location

4. drive data (/Users/mac/Library/Developer/Xcode/DerivedData)

5. delete all to trash.

// if cleaning derived data didn't help

- Restart your mac or fixed this issue by

1. deleted Xcode

2. updating my Mac to MacOS High Siera

3. installed new / latest / low version ex: Xcode 9.3  from AppStore 

- Hope to help.

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