问题
what is the actual root cause of this issue? I already sync with gradle files but eventually failed.

回答1:
Just sync your project with gradles. File --> Sync Project with Gradle Files
回答2:
- Build -> Clean Project
- Build -> Rebuild Project
File -> Sync project with gradle files . If not worked then try
File -> Invalidate Caches / Restart .
It's work for me !!!
回答3:
That could be fixed in one or two steps, depending on how nice is Android Studio today:
- Try to sync your project with gradle files:
- If that doesn't work, invalidate Android Studio caches and restart. That should fix the problem:
The reason behind the first solution not working could be related on gradle cached dependencies problems, but who knows 🤷
回答4:
Issue happened to me when I changed the res>values>styles.xml
What I change was"Theme.AppCompat.Light.DarkActionBar" --->>> "Base.Theme.AppCompat.Light.DarkActionBar"
Solutions I tried.
- Build -> Rebuild Project (Not worked)
- Close all files and Open again. Then Build -> Clean Project (Not worked) File -> Sync project with gradle files (Not worked)
- Close the Android Studio while project still opened. Opened it again. - It Worked.
回答5:
Just you have to do >> open File --> Sync Project with Gradle Files and then after >> open File --> invalidate Android Studio caches / restart so done this problem.:)
回答6:
If nothing of the above steps work, Then simply undo the changes that you have recently made, and then build gradle. In my case, I have added one dependencies and this problem raised, so I removed that dependencies build the gradle and then again add it. Problem Solved..!!
来源:https://stackoverflow.com/questions/53195701/android-studio-design-editor-is-unavailable-until-after-a-successful-project-syn