backends/p4android/client: initialize __callbacks to None

Initialize __callbacks to None in the __init__ method. This avoids an
AttributeError when reading the is_connected property before the client
connects.

Fixes: #1791

Author: dlech

CHANGELOG.rst

@@ -10,6 +10,11 @@ and this project adheres to `Semantic Versioning <https://semver.org/spec/v2.0.0
 `Unreleased`_
 =============
 
+Fixed
+-----
+- Fixed ``AttributeError`` in Python4Android backend when accessing ``is_connected`` before connecting. Fixes #1791.
+
+
 `1.1.0`_ (2025-08-10)
 =====================
 

bleak/backends/p4android/client.py

@@ -61,6 +61,8 @@ def __init__(
         self.__gatt = None
         self.__mtu = 23
 
+        self.__callbacks = None
+
     # Connectivity methods
 
     @override