cellular: for IP XBees do not retrieve network callbacks

Signed-off-by: Tatiana Leon <tatiana.leon@digi.com>

Author: tatianaleon

digi/xbee/devices.py

@@ -3052,6 +3052,9 @@ def get_xbee_device_callbacks(self):
         """
         api_callbacks = PacketReceived()
 
+        if not self._network:
+            return api_callbacks
+
         for i in self._network.get_discovery_callbacks():
             api_callbacks.append(i)
         return api_callbacks