Extra guarding of channel, as suggested

Author: bouwew

plugwise_usb/__init__.py

@@ -94,6 +94,11 @@ def cache_enabled(self, enable: bool = True) -> None:
     @property
     def channel(self) -> int | None:
         """Zigbee channel number."""
+        if (
+            not self._controller.is_connected
+            or self._network is None
+        ):
+            return None
         return self._network.channel
 
     @property