fix: Ensure self.tail_thread to be set before add_close_hook

The self.tail_thread attrbuite must to be set before adding the
Tail._join_thread to prevent unable to use it on cleanup, otherwise
sometimes will raise AttributeError.

Signed-off-by: Yihuang Yu <[email protected]>

aexpect/client.py

@@ -536,6 +536,7 @@ def __init__(self, command=None, a_id=None, auto_close=False, echo=False,
         :param encoding: Override text encoding (by default: autodetect by
                 locale.getpreferredencoding())
         """
+        self.tail_thread = None
         # Add a reader and a close hook
         self._add_reader("tail")
         self._add_close_hook(Tail._join_thread)
@@ -556,7 +557,6 @@ def __init__(self, command=None, a_id=None, auto_close=False, echo=False,
         self.output_prefix = output_prefix
 
         # Start the thread in the background
-        self.tail_thread = None
         if self.is_alive():
             if termination_func or output_func:
                 self._start_thread()