Fix editor resource browser preview camera position and failure to assign model by drag & drop. Closes #1848.

bin/Data/Scripts/Editor/EditorActions.as

@@ -1059,8 +1059,8 @@ class AssignModelAction : EditAction
     void Define(StaticModel@ staticModel_, Model@ oldModel_, Model@ newModel_)
     {
         staticModel = staticModel_;
-        oldModel = oldModel_.name;
-        newModel = newModel_.name;
+        oldModel = (oldModel_ !is null) ? oldModel_.name : "";
+        newModel = (newModel_ !is null) ? newModel_.name : "";
     }
 
     void Undo()

bin/Data/Scripts/Editor/EditorResourceBrowser.as

@@ -838,7 +838,7 @@ void HandleResourceBrowserFileListSelectionChange(StringHash eventType, VariantM
                 if (boxes[i].size.length > biggestBox.size.length)
                     biggestBox = boxes[i];
             }
-            resourcePreviewCameraNode.position = biggestBox.center + camPosition * biggestBox.size.length;
+            resourcePreviewCameraNode.position = camPosition * biggestBox.size.length + biggestBox.center;
         }
 
         resourcePreviewScene.AddChild(resourcePreviewNode);