[FIXED HUDSON-2527] Fixed NPE and instead deal with this gracefully.

Improved doc to call out for this error-prone definition of semantics.

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@12850 71c3de6d-444a-0410-be80-ed276b4c234a

core/src/main/java/hudson/Functions.java

@@ -866,6 +866,7 @@ public class Functions {
      */
     public static String getActionUrl(String itUrl,Action action) {
         String urlName = action.getUrlName();
+        if(urlName==null)   return null;    // to avoid NPE and fail to render the whole page
 
         if(SCHEME.matcher(urlName).matches())
             return urlName; // absolute URL

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

@@ -71,7 +71,8 @@ public interface Action extends ModelObject {
      * which is useful for directly connecting to external systems.
      *
      * @return
-     *      null if this action object doesn't need to be bound to web.
+     *      null if this action object doesn't need to be bound to web
+     *      (when you do that, be sure to also return null from {@link #getIconFileName()}.
      */
     String getUrlName();
 }