I'm getting null query parameter string even though passing query parameter string like this login?redirect=%2Farticles%2Fadd.
I have tried with $this->request->getQuery('redirect'); and $this->request->query('redirect'); but no luck.
Does anyone have any idea that where it would be wrong?
Looks like you are not setup your URL Route properly in your config/routes.php file. When you are interested to pass any route parameter to your controller action you should configure your route proper way. Although your question is not written detailedly.
I hope you will get much more information from this articles.