Updates an error msg for media resource, performs a null check

src/Umbraco.Web.UI.Client/src/common/resources/mediatype.resource.js

@@ -34,7 +34,7 @@ function mediaTypeResource($q, $http, umbRequestHelper) {
                        "mediaTypeApiBaseUrl",
                        "GetAllowedChildren",
                        [{ contentId: mediaId }])),
-               'Failed to retrieve data for media id ' + mediaId);
+               'Failed to retrieve allowed types for media id ' + mediaId);
         }
 
     };

src/Umbraco.Web/Editors/ContentControllerBase.cs

@@ -154,9 +154,11 @@ namespace Umbraco.Web.Editors
         /// </remarks>
         protected TPersisted GetObjectFromRequest<TPersisted>(Func<TPersisted> getFromService)
         {
-            return Request.Properties.ContainsKey(typeof (TPersisted).ToString()) == false
-                       ? getFromService()
-                       : (TPersisted) Request.Properties[typeof (TPersisted).ToString()];
+            //checks if the request contains the key and the item is not null, if that is the case, return it from the request, otherwise return
+            // it from the callback
+            return Request.Properties.ContainsKey(typeof(TPersisted).ToString()) && Request.Properties[typeof(TPersisted).ToString()] != null
+                ? (TPersisted) Request.Properties[typeof (TPersisted).ToString()]
+                : getFromService();
         } 
 
         /// <summary>

src/Umbraco.Web/Editors/MediaController.cs

@@ -96,6 +96,8 @@ namespace Umbraco.Web.Editors
             if (foundContent == null)
             {
                 HandleContentNotFound(id);
+                //HandleContentNotFound will throw an exception
+                return null;
             }
             return Mapper.Map<IMedia, MediaItemDisplay>(foundContent);
         }