MissingControllerException Error

I recently updated to cake php 3.6.5 from 3.2.6 using composer.
After the update, my application seems to be breaking. My custom error handler that I have used extending ErrorHandler is failing. Apart from this, my first controller is not found by the cake php. It says -Error: Create the class XXXXController below in file: src/Controller/XXXXController.php

You should check the migration guide before upgrading cake, there are alot of changes from 3.2 to 3.6: