[FIXED JENKINS-21875]

User.current() can return null, so this better check for null

changelog.html

@@ -61,6 +61,9 @@ Upcoming changes</a>
   <li class=bug>
     Fixed a slow down in resource loading caused by fix to JENKINS-18677.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-21579">issue 21579</a>)
+  <li class=bug>
+    Fixed NPE in <tt>UserCause</tt>
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-21875">issue 21875</a>)
 </ul>
 </div><!--=TRUNK-END=-->
 

core/src/main/java/hudson/slaves/OfflineCause.java

@@ -109,7 +109,7 @@ public abstract class OfflineCause {
 
         public UserCause(User user, String message) {
             super(hudson.slaves.Messages._SlaveComputer_DisconnectedBy(
-                    user.getId(),
+                    user!=null ? user.getId() : Jenkins.ANONYMOUS.getName(),
                     message != null ? " : " + message : ""
             ));
             this.user = user;