I have updated my android studio from 3.5.x to 3.6 today and getting error while generating signed apk for build variant showing the follow
Problems encountered when upgrading to the latest Android studio 3.6.1
Clearing the cache, clean project, and deleting the build folder did not solve the problem.
While generating the apk, check for the destination folder, from which delete output.json and app-debug.apk from debug folder. Try building the same after this. AFAIK, it is a bug for android studio, hope they might fix this soon :)