I'd like to tag a build, but I receive the following exception. I've done it
before, but I've upgraded to v1.191 since then (which I believe was 1.181).

HTTP Status 500 -

type Exception report

message

description The server encountered an internal error () that prevented it from
fulfilling this request.

exception

javax.servlet.ServletException: java.lang.NullPointerException
org.kohsuke.stapler.Stapler.invoke(Stapler.java:370)
org.kohsuke.stapler.MetaClass$15.dispatch(MetaClass.java:362)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$15.dispatch(MetaClass.java:362)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:241)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:241)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:298)
org.kohsuke.stapler.Stapler.service(Stapler.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:88)

root cause

java.lang.NullPointerException
hudson.scm.AbstractScmTagAction.doIndex(AbstractScmTagAction.java:38)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
java.lang.reflect.Method.invoke(Method.java:597)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:103)
org.kohsuke.stapler.MetaClass$5.dispatch(MetaClass.java:175)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$15.dispatch(MetaClass.java:362)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$15.dispatch(MetaClass.java:362)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:241)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.MetaClass$9.doDispatch(MetaClass.java:241)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:298)
org.kohsuke.stapler.Stapler.service(Stapler.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:88)