Q:

Android Studio Theme editor not showing previews? 

I am doing android development.  I could not solve this problem. 

I've tried making a new project and see what is happening there, but it's still not showing any preview. 
I feel like I'm missing something silly. 

A:

I have answered this problem in this post: 
After Updating Android studio to 3.1.2 , I get "Failed to load AppCompat ActionBar with unknown error. " 
It is some kind of bug.  I had the same problem.  I searched so much and I finally found that appcompat-v7:28.0.0-alpha3 has some bug with "Design View" part of Android Studio. 
So I suggest to change com.android.support:appcompat-v7:28.0.0-alpha3 to com.android.support:appcompat-v7:28.0.0-alpha1 version in the build.gradle (Module:app) and then everything is OK. 
Of course you should have internet access to download com.android.support:appcompat-v7:28.0.0-alpha1

