Bluetooth + Flutterflow

Hi experts,

i followed the tutorial to connect my ESP32 micro controller to my Flutterflow but I am struggeling to see the data from my ESP32 device in my flutterflow app.

So here is the shortened code on my ESP32 device

def register(self):        
        # Nordic UART Service (NUS)
        NUS_UUID = '6E400001-B5A3-F393-E0A9-E50E24DCCA9E'
        RX_UUID = '6E400002-B5A3-F393-E0A9-E50E24DCCA9E'
        TX_UUID = '6E400003-B5A3-F393-E0A9-E50E24DCCA9E'
            
        BLE_NUS = ubluetooth.UUID(NUS_UUID)
        BLE_RX = (ubluetooth.UUID(RX_UUID), ubluetooth.FLAG_WRITE)
        BLE_TX = (ubluetooth.UUID(TX_UUID), ubluetooth.FLAG_READ | ubluetooth.FLAG_NOTIFY)
            
        BLE_UART = (BLE_NUS, (BLE_TX, BLE_RX,))
        SERVICES = (BLE_UART, )
        ((self.tx, self.rx,), ) = self.ble.gatts_register_services(SERVICES)

def send(self, data):
        if self.connected_flag:
            self.ble.gatts_notify(0, self.tx, data + '\n')

while True:
    if ble.connected_flag:
        for n in range(1,100):
            ble.send(str(n))
            sleep_ms(1000)

I do see results on my phone if I use nRF Connect:

But I do not see anything in the Flutterflow app although my device is connected:

Here is the code that I use in my custom action "receiveData":

// Automatic FlutterFlow imports
import '/backend/schema/structs/index.dart';
import '/flutter_flow/flutter_flow_theme.dart';
import '/flutter_flow/flutter_flow_util.dart';
import '/custom_code/actions/index.dart'; // Imports other custom actions
import '/flutter_flow/custom_functions.dart'; // Imports custom functions
import 'package:flutter/material.dart';
// Begin custom action code
// DO NOT REMOVE OR MODIFY THE CODE ABOVE!

import 'package:flutter_blue_plus/flutter_blue_plus.dart';

Future<String?> receiveData(BTDeviceStruct deviceInfo) async {
  final device = BluetoothDevice.fromId(deviceInfo.id, name: deviceInfo.name);
  final services = await device.discoverServices();
  for (BluetoothService service in services) {
    for (BluetoothCharacteristic characteristic in service.characteristics) {
      final isRead = characteristic.properties.read;
      final isNotify = characteristic.properties.notify;
      if (isRead && isNotify) {
        final value = await characteristic.read();
        return String.fromCharCodes(value);
      }
    }
  }
  return null;
}

I have the feeling that the reason is that this custom action is called every second on page initialization and that therewith the sending device and the receiving phone are out of sync and never "meet" each other.

Any idea how to solve it?