UserRoleActorFactory is not null-safe causing weird behaviour at some point, e.g. comment box in view issue screen

A scenario which we discovered from a user's support request:

1. Enable anonymous comment (Add "Anyone" to "Add Comment" and "Browse Project" permission)
2. Ensure that users are added into the "Administrators" role in the project role mapping, e.g. Users: Foo, Bar; Groups: jira-administrators
3. View issue without logging into JIRA and you will notice two things:
   1. NPE thrown and logged in the log file:

      2007-06-22 18:33:12,937 http-8090-Processor4 ERROR [webwork.util.ServletValueStack] METHOD: "createHtml", exception: 
      java.lang.NullPointerException
      	at com.atlassian.jira.security.roles.actor.UserRoleActorFactory$UserRoleActor.contains(UserRoleActorFactory.java:114)
      	at com.atlassian.jira.security.roles.CachingProjectRoleAndActorStore$CachedDefaultRoleActors.contains(CachingProjectRoleAndActorStore.java:465)
      	at com.atlassian.jira.security.roles.DefaultProjectRoleManager.getProjectRoles(DefaultProjectRoleManager.java:37)
      	at com.atlassian.jira.issue.fields.CommentSystemField.getRoleLevels(CommentSystemField.java:515)
      	at com.atlassian.jira.issue.fields.CommentSystemField.populateVelocityParams(CommentSystemField.java:407)
      	at com.atlassian.jira.issue.fields.CommentSystemField.getCreateHtml(CommentSystemField.java:108)
      	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      

   2. Comment fields disappeared (although the "Add Comment" container is visible)

This happens because both UserRoleActorFactory and GroupRoleActorFactory are added to the CachedDefaultRoleActors as its construction level.