In the microcontroller system, we read the data from the DHT-11 module from digital pin 2. And we convert this numeric value to string type. We convert this new string type data into an array-char type value. This value is sent to the Bluetooth module with the myserial.write command.

On the Android application side, we get the value of this char type with the receiveData function. This function is included in the timer function.
 At certain time intervals, this value is read and written to the text object.
  

Flutter-Dart codes are given below:

import 'dart:async';
import 'dart:convert';
 //import 'dart:convert';

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
 import 'package:flutter_bluetooth_serial/flutter_bluetooth_serial.dart';

void main()=> runApp(const MyApp());

class MyApp extends StatefulWidget {
   const MyApp({super.key});

  @override
  // ignore: library_private_types_in_public_api
  _MyAppState createState()=> _MyAppState();
 }

class _MyAppState extends State {
  List _devices=[];
  late BluetoothConnection connection;
  String adr="00:21:07:00:50:69"; // my bluetooth device MAC Adres
   String recData="....";

  late Timer _timer;
   String _timeString="";

  @override
  void initState() {
    // _startTimer();  automatic
    super.initState();
    _loadDevices();
   }

  Future _loadDevices() async {
    List devices=await FlutterBluetoothSerial.instance
         .getBondedDevices();

    setState(() {
      _devices=devices;
    });
   }

  //----------------------------
  Future sendData(String data) async {
    data=data.trim();
    try {
      List list=data.codeUnits;
      Uint8List bytes=Uint8List.fromList(list);
      connection.output.add(bytes);
      await connection.output.allSent;
      if (kDebugMode) {
        // print('Data sent successfully');
      }
    } catch (e) {
      //print(e.toString());
    }
   }

  // data RECEIVED --------------

  Future receiveData() async {
    connection.input!.listen((Uint8List data) {
      //Data entry point
      setState(() {
        recData=ascii.decode(data);
        //var n1=int.parse('-42');
      });
    });
  }
   //--------------------------------------

// TIMER START-----------
  void _startTimer() {
    _timer=Timer.periodic(Duration(milliseconds: 500), (timer) {
       receiveData();

    });
   }

  // TIMER STOP--------------------------------------
  Future _stopTimer() async {
    setState(() {
    });
    _timer.cancel();
   }

//---------------------------------------------
  @override
  void dispose() {
    _timer.cancel();
    super.dispose();
   }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        home: Scaffold(
          appBar: AppBar(
            title: const Text("----Temperature and Humidity Meter with BlueTooth-----"),
          ),
          body: Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                 const Text("MAC Adress: 00:21:07:00:50:69"),

                ElevatedButton(child: Text("Connect"), onPressed: () {
                  connect(adr);
                 },),

                const SizedBox(height: 30.0,),

                const Text("Temperature-Humidity: ",style: TextStyle(fontSize: 55.0),),
                 Text(recData,style: TextStyle(fontSize: 40.0),),

                const SizedBox(height: 10.0,),
                // Text(_timeString),
                 const SizedBox(height: 10.0,),

                ElevatedButton(child: Text("Stop timer"), onPressed: () {
                  _stopTimer();
                },),
                 const SizedBox(height: 10.0,),

                ElevatedButton(child: Text("Start timer"), onPressed: () {
                  _startTimer();
                 },),

              ],
            ),
           ),

        )

    );
   }

  Future connect(String address) async {
    try {
      connection=await BluetoothConnection.toAddress(address);
      // sendData('111');
       //durum="Connected to the device";

    } catch (exception) {
      // durum="Cannot connect, exception occured";
    }
   }

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
   }

// --------------**************data gonder
//Future send(Uint8List data) async {
//connection.output.add(data);
// await connection.output.allSent;
 // }

}
 //------------*********** data gonder end

Microcontroller codes-Arduino:

#include 
#include  

SoftwareSerial mySerial(10, 11); // RX, TX
#define DHT11PIN 2 //  

char Buf[100]; //  to bluetooth device<------
String values_string="--";

dht11 DHT11;

void setup()
{
  Serial.begin(9600); // 
  mySerial.begin(9600);
  
}

void loop()
{
  
               int chk=DHT11.read(DHT11PIN);
    
  float humd=(float)DHT11.humidity;
  float temp=(float)DHT11.temperature;
  int fahr=DHT11.fahrenheit();
  int kelv=DHT11.kelvin();
    
  
  values_string="Humd:%"+String(humd)+"  Temp:" + String(temp)+"C  Fahr:" + String(fahr)+"F  Kelv:"+ String(kelv)+"K"; 
     
     values_string.toCharArray(Buf, 100);
     mySerial.write(Buf); 

     Serial.println(values_string);
  delay(1000);
  
}