catch if cm_config of other key undefined

IPython/html/static/notebook/js/cell.js

@@ -323,7 +323,12 b' var IPython = (function (IPython) {'
             }
         }
         // fallback on default
-        var default_mode = this._options.cm_config.mode || 'text/plain';
+        var default_mode
+        try {
+            default_mode = this._options.cm_config.mode;
+        } catch(e) {
+            default_mode = 'text/plain';
+        }
         this.code_mirror.setOption('mode', default_mode);
     };