Helllo @AparnaKochar
System.Web.HttpContext.Current is populated by IIS which is not present/active during unit testing. Hence null. Do not tightly couple your code to HttpContext for that very reason. Instead encapsulate that behind abstractions that can be mocked when testing in isolation.
Bad design aside, for your specific design you are trying to access a static dependency external to the controller. If faking the controller's context as demonstrated in your current test, then access the controller's context instead of calling the static context.
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