Dont err if view name isn't registered.

IPython/html/static/notebook/js/widget.js

@@ -212,17 +212,20 b' define(["components/underscore/underscore-min",'
                     var parent_view = parent_views[parent_view_index];
                     if (parent_view.display_child != undefined) {
                         var view = this._create_view(view_name, cell);
+                        if (view != null) {
                         new_views.push(view);
                         parent_view.display_child(view);
                         displayed = true;
                     }
                 }
             }
+            }
 
             if (!displayed) {
                 // No parent view is defined or exists.  Add the view's 
                 // element to cell's widget div.
                 var view = this._create_view(view_name, cell);
+                if (view != null) {
                 new_views.push(view);
 
                 if (cell.widget_subarea != undefined && cell.widget_subarea != null) {
@@ -230,6 +233,7 b' define(["components/underscore/underscore-min",'
                     cell.widget_subarea.append(view.$el);
                 }
             }
+            }
 
             for (var view_index in new_views) {
                 var view = new_views[view_index];
@@ -240,7 +244,9 b' define(["components/underscore/underscore-min",'
 
         // Create a view
         _create_view: function (view_name, cell) {
-            var view = new this.widget_manager.widget_view_types[view_name]({model: this});
+            var view_type = this.widget_manager.widget_view_types[view_name]; 
+            if (view_type != undefined && view_type != null) {
+                var view = new view_type({model: this});
             view.render();
             if (this.views[cell]==undefined) {
                 this.views[cell] = []
@@ -266,6 +272,8 b' define(["components/underscore/underscore-min",'
                 }
             });
             return view;
+            }
+            return null;
         },