Fall back on new kernel if kernel does note exist

notebook/services/sessions/handlers.py

@@ -58,7 +58,8 @@ class SessionRootHandler(APIHandler):
         else:
             try:
                 model = yield gen.maybe_future(
-                    sm.create_session(path=path, kernel_name=kernel_name))
+                    sm.create_session(path=path, kernel_name=kernel_name,
+                                      kernel_id=kernel_id))
             except NoSuchKernel:
                 msg = ("The '%s' kernel is not available. Please pick another "
                        "suitable kernel instead, or install that kernel." % kernel_name)

notebook/services/sessions/sessionmanager.py

@@ -67,9 +67,8 @@ class SessionManager(LoggingConfigurable):
     def create_session(self, path=None, kernel_name=None, kernel_id=None):
         """Creates a session and returns its model"""
         session_id = self.new_session_id()
-        if kernel_id is not None:
-            if kernel_id not in self.kernel_manager:
-                raise web.HTTPError(400, "No such kernel: %s" % kernel_id)
+        if kernel_id is not None and kernel_id in self.kernel_manager:
+            pass
         else:
             kernel_id = yield self.start_kernel_for_session(session_id, path,
                                                             kernel_name)