fix NimBLE notify characteristic registration

main/trikke_ble_transport.c

@@ -45,6 +45,21 @@ static uint32_t get_u32_le(const uint8_t *input)
            ((uint32_t)input[2] << 16) | ((uint32_t)input[3] << 24);
 }
 
+static int data_access(
+    uint16_t connection_handle,
+    uint16_t attribute_handle,
+    struct ble_gatt_access_ctxt *context,
+    void *argument)
+{
+    (void)connection_handle;
+    (void)attribute_handle;
+    (void)context;
+    (void)argument;
+    // NimBLE requires every characteristic definition to have an access
+    // callback, even though this notify-only value is never client-accessible.
+    return BLE_ATT_ERR_UNLIKELY;
+}
+
 static int ack_access(
     uint16_t connection_handle,
     uint16_t attribute_handle,
@@ -94,6 +109,7 @@ static const struct ble_gatt_svc_def TRIKKE_GATT_SERVICES[] = {
         .characteristics = (struct ble_gatt_chr_def[]) {
             {
                 .uuid = &TRIKKE_DATA_UUID.u,
+                .access_cb = data_access,
                 .flags = BLE_GATT_CHR_F_NOTIFY,
                 .val_handle = &s_data_value_handle,
             },