Move a call to RequestModuleInterface to a new scene callback function

because the current implementation would have always returned null

OpenSim/ApplicationPlugins/RegionModulesController/RegionModulesControllerPlugin.cs

@@ -509,6 +509,8 @@ namespace OpenSim.ApplicationPlugins.RegionModulesController
             {
                 module.RegionLoaded(scene);
             }
+
+            scene.AllModulesLoaded();
         }
 
         public void RemoveRegionFromModules (Scene scene)

OpenSim/Region/Framework/Scenes/Scene.cs

@@ -1243,13 +1243,21 @@ namespace OpenSim.Region.Framework.Scenes
             get { return m_sceneGraph; }
         }
 
-        protected virtual void RegisterDefaultSceneEvents()
+        /// <summary>
+        /// Called by the module loader when all modules are loaded, after each module's
+        /// RegionLoaded hook is called. This is the earliest time where RequestModuleInterface
+        /// may be used.
+        /// </summary>
+        public void AllModulesLoaded()
         {
             IDialogModule dm = RequestModuleInterface<IDialogModule>();
 
             if (dm != null)
                 m_eventManager.OnPermissionError += dm.SendAlertToUser;
+        }
 
+        protected virtual void RegisterDefaultSceneEvents()
+        {
             m_eventManager.OnSignificantClientMovement += HandleOnSignificantClientMovement;
         }