After installing this plugin, when you tried to add credentials, you will see an empty/blank credential (or a credential that will be marked as "null") in the credential selection dropdown.

If you select this and then try to save, you will see a NPE whose call stack's top frames look like this:

javax.servlet.ServletException: java.lang.NullPointerException
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:796)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.MetaClass$13.dispatch(MetaClass.java:411)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:746)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:876)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:649)
at org.kohsuke.stapler.Stapler.service(Stapler.java:238)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:723)

This does not affect the usage of this plugin. The credential provided by this plugin should not appear in "add credentials" dropdown. Just don't select this empty/blank/null credential in "add credentials" dropdown. Follow the instructions on the plugin's wiki page.