I tried hard to install / configure Flagged Revs on Mediawiki I got different errors such as :
After gave the current user (bureaucratic) to Reviewer suddenly the whole wiki stops working
Then I start with fresh installation of media wiki and did it again, sometime it happens unknowingly we configure some wrong option, and they guy like me got into trouble :).
Anyhow when I investigate it further the php_error log file, following error was found.
Call to undefined method ObjectCache::getMainStashInstance() in C:\wamp\www\newwiki\extensions\FlaggedRevs\backend\FRUserActivity.php on line 56
With the help of Krenair, figure it out that I've incompatible version of FlaggedRev and MediaWiki, so I download the mediawiki-1.26.2 with FlaggedRevs-REL1_26-56a565c.
The problem solve!!
Actually, as error states that getMainStashInstance is not available, so when I checked in media wiki 1.24 the function implementation was not available and Krenair helped me to figure it out in which version it will be available, lets cut it short, use the latest version of both Flagged and Mediawiki to avoid such issues.