pybricks.messaging.BLERadio: Fix observe not working if hub instantiated later.

The backwards compatibility condition was correctly caught, but the radio would re-initialize anyway.

Author: laurensvalk

CHANGELOG.md

@@ -6,6 +6,12 @@
 
 [Unreleased]: https://github.com/pybricks/pybricks-micropython/compare/v4.0.0b11...HEAD
 
+### Fixed
+- Fixed `BLERadio.observe()` not working if a hub was instantiated
+  after `BLERadio` ([support#2734]).
+
+[support#2734]: https://github.com/pybricks/support/issues/2734
+
 ## [4.0.0] - 2026-06-08
 
 ### Changed

pybricks/messaging/pb_type_ble_radio.c

@@ -586,8 +586,9 @@ static mp_obj_t pb_type_ble_radio_make_new(const mp_obj_type_t *type, size_t n_a
 mp_obj_t pb_type_BLE_new(mp_obj_t broadcast_channel_in, mp_obj_t observe_channels_in) {
     if (broadcast_channel_in != mp_const_none || observe_channels_in != MP_OBJ_FROM_PTR(&mp_const_empty_tuple_obj)) {
         mp_printf(&mp_plat_print, "Hub messaging has been moved. You should use:\n\nfrom pybricks.messaging import BLERadio\nradio = BLERadio(broadcast_channel, observe_channels)\n\n");
+        return pb_type_ble_radio_init(broadcast_channel_in, observe_channels_in);
     }
-    return pb_type_ble_radio_init(broadcast_channel_in, observe_channels_in);
+    return mp_const_none;
 }
 #endif // PYBRICKS_PY_MESSAGING_BLE_RADIO_OLD