The back button problem is a bug in the Tizen source code.
I've posted this question in the Tizen developer forums (https://developer.tizen.org/forums/general-support/back-button-not-working-on-tizen-rd-pq-reference-device) , where I got the answer at the following link.
https://developer.tizen.org/forums/general-support/latest-image-rd-pq https://developer.tizen.org/forums/general-support/back-buttons
If anyone faces the similar issue with the back button they can follow this forum topic for the work around to this problem.