Hello Kumar,
in the xsaccess file you have to set an authentication mechanism instead of "null".
For example (to allow Basic and Form based authentication):
"authentication" : [
{"method" : "Basic"},
{"method" : "Form"}
]
Please check also in the Admin tool (<host>:<port>/sap/hana/xs/admin) what authentication types you have allowed for the application.
Best Regards,
Florian