Merge pull request #61 from golf-player/master

Fixing AttributeError when kernel cleanup occurs before the kernel is created.

nbclient/client.py

@@ -467,7 +467,7 @@ async def async_execute(self, **kwargs):
  The executed notebook.
  """
  reset_kc = kwargs.get('reset_kc', False)
- if reset_kc:
+ if reset_kc and self.km:
  await self._async_cleanup_kernel()
  self.reset_execution_trackers()