SPSecurityTokenService.Issue() failed: System.NullReferenceException: Object reference not set to an instance of an object

Basically it is looking for alternate access mapping. Below steps for creating alternate access mapping

Login to Central Admin.

1.            Go to Application Management.

2.            Under Web Applications click Configure alternate access mappings.

3.            Click on Add Internal URLs link on the tool bar. If you want to configure intranet or internet url’s click on Edit Public URLs

4.            In the URL protocol, host and port text box, type or verify the full URL of your site that is using the FBA Authentication.

5.            Set Zone to Default and save.