[JENKINS-12302]

Make sure that we don't require Jenkins.getInstance() on CLICommand, since it can run on slaves.

core/src/main/java/hudson/cli/CLICommand.java

@@ -472,7 +472,9 @@ public abstract class CLICommand implements ExtensionPoint, Cloneable {
     static {
         // register option handlers that are defined
         ClassLoaders cls = new ClassLoaders();
-        cls.put(Jenkins.getInstance().getPluginManager().uberClassLoader);
+        Jenkins j = Jenkins.getInstance();
+        if (j!=null) {// only when running on the master
+            cls.put(j.getPluginManager().uberClassLoader);
 
             ResourceNameIterator servicesIter =
                 new DiscoverServiceNames(cls).findResourceNames(OptionHandler.class.getName());
@@ -485,4 +487,5 @@ public abstract class CLICommand implements ExtensionPoint, Cloneable {
                 CmdLineParser.registerHandler(c,h);
             }
         }
+    }
 }