NPE when NodeMonitor.columnCaption is null.

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

@@ -214,7 +214,10 @@ public final class ComputerSet extends AbstractModelObject implements Describabl
         
         for (NodeMonitor nodeMonitor : NodeMonitor.getAll()) {
             Thread t = nodeMonitor.triggerUpdate();
-            t.setName(nodeMonitor.getColumnCaption());
+            String columnCaption = nodeMonitor.getColumnCaption();
+            if (columnCaption != null) {
+                t.setName(columnCaption);
+            }
         }
         rsp.forwardToPreviousPage(req);
     }

core/src/main/java/hudson/node_monitors/NodeMonitor.java

@@ -36,6 +36,7 @@ import hudson.model.Descriptor;
 import hudson.util.DescriptorList;
 
 import java.util.List;
+import javax.annotation.CheckForNull;
 
 import org.kohsuke.stapler.export.Exported;
 import org.kohsuke.stapler.export.ExportedBean;
@@ -75,7 +76,7 @@ public abstract class NodeMonitor implements ExtensionPoint, Describable<NodeMon
      *      null to not render a column. The convention is to use capitalization like "Foo Bar Zot".
      */
     @Exported
-    public String getColumnCaption() {
+    public @CheckForNull String getColumnCaption() {
         return getDescriptor().getDisplayName();
     }