After upgrade Android Studio, i see below issue with Cross mark, And When I click on Edit Configuration, I see as below Default Activity not found. Thus not allowing to run app.

I tried different solutions like

• File -> Invalidate Cache/ Restart,
• Build -> Clean build, 
• File -> Sync Project With gradle files and
• Delete files inside C:/User/praveenb/.gradle/caches.

Nothing worked, Finally, I tried Exit Android Studio and  delete files inside “C:\Users\Praveenb\.AndroidStudio4.0\system\caches”. And then open Android Sutudio, solved the “Default activity not found” issue.

This fix also, solved other issue in, Previously, whenever i open AndroidManifest file -> Meged manifest tab, It used to show 

Unable to compute merged manifest 

This issue also solved by doing the above step.

Hope it helps somebody.