fixed NPE.

See https://github.com/jenkinsci/jenkins/pull/474 for the original conversation

core/src/main/java/hudson/model/View.java

@@ -31,6 +31,7 @@ import hudson.Indenter;
 import hudson.Util;
 import hudson.model.Descriptor.FormException;
 import hudson.model.Node.Mode;
+import hudson.model.labels.LabelAtom;
 import hudson.model.labels.LabelAtomPropertyDescriptor;
 import hudson.scm.ChangeLogSet.Entry;
 import hudson.search.CollectionSearchIndex;
@@ -67,6 +68,7 @@ import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
+import java.util.Set;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 
@@ -383,7 +385,7 @@ public abstract class View extends AbstractModelObject implements AccessControll
         for (Computer c : computers) {
             Node n = c.getNode();
             if (n != null) {
-                if (labels.contains(null) && n.getMode() == Mode.NORMAL || !Collections.disjoint(n.getAssignedLabels(), labels)) {
+                if (labels.contains(null) && n.getMode() == Mode.NORMAL || !isDisjoint(n.getAssignedLabels(), labels)) {
                     result.add(c);
                 }
             }
@@ -391,7 +393,14 @@ public abstract class View extends AbstractModelObject implements AccessControll
 
         return result;
     }
-    
+
+    private boolean isDisjoint(Collection c1, Collection c2) {
+        for (Object o : c1)
+            if (c2.contains(o))
+                return false;
+        return true;
+    }
+
     public List<Queue.Item> getQueueItems() {
         if (!isFilterQueue()) {
             return Arrays.asList(Jenkins.getInstance().getQueue().getItems());