Fix OutStream using _fid before being defined (#1373)

Author: davidbrochart

ipykernel/iostream.py

@@ -388,6 +388,9 @@ def _watch_pipe_fd(self):
 
         """
 
+        if self._fid is None:
+            return
+
         try:
             bts = os.read(self._fid, PIPE_BUFFER_SIZE)
             while bts and self._should_watch:
@@ -441,6 +444,7 @@ def __init__(
         self.session = session
         self._has_thread = False
         self.watch_fd_thread = None
+        self._fid = None
         if not isinstance(pub_thread, IOPubThread):
             # Backward-compat: given socket, not thread. Wrap in a thread.
             warnings.warn(