handle faulty extensions

vispa/static/js/extensions.js

@@ -115,6 +115,9 @@ define(["jquery", "emitter", "vispa/module", "vispa/ui"], function($, Emitter, M
     registerExtension: function(extension) {
       var self = this;
 
+      if (extension === undefined)
+        return null;
+      
       if (!(extension instanceof Extension)) {
         if (extension._extends(Extension))
           extension = new extension();