Check for null rather than undefined

set kernel id to None/null if not started

IPython/frontend/html/notebook/handlers.py

@@ -593,10 +593,7 class NotebookRootHandler(AuthenticatedHandler):
         km = self.application.kernel_manager
         files = nbm.list_notebooks()
         for f in files :
-            nid = f['notebook_id']
-            kid = km.kernel_for_notebook(nid)
-            if  kid is not None:
-                f['kernel_id'] = kid
+            f['kernel_id'] = km.kernel_for_notebook(f['notebook_id'])
         self.finish(jsonapi.dumps(files))
 
     @web.authenticated

IPython/frontend/html/notebook/static/js/notebooklist.js

@@ -113,7 +113,7 var IPython = (function (IPython) {
             this.add_link(notebook_id, nbname, item);
             if (!IPython.read_only){
                 // hide delete buttons when readonly
-                if(kernel == undefined){
+                if(kernel == null){
                     this.add_delete_button(item);
                 } else {
                     this.add_shutdown_button(item,kernel);