Merge pull request #61 from golf-player/master

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

Author: davidbrochart

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()