I find out the solution. The last widget sticky code is working now.
Problem: I have 2 widgets 1st one is Text-Based Widget with Google AdSense Code and the Ad unit was responsive. So, the problem was responsive Ad Unit. Solution: While blocking the ads in my site using the AdBlocker, the sticky widget was working.
So, for that, I changed the responsive Ad Unit to Fixed Size Ad Unit. And now the sticky code is working.