Null check operator used on a null value when calling a function from another class – Flutter

I have a class called DataCollectionPage where I have the following function in the initState function:

void establishConnection (){
    BluetoothConnection.toAddress(widget.server!.address).then((connection) {
      Utils.showSnackBar2("Successfully connected to your device");
      this.connection = connection;
      setState(() {
        isConnecting = false;
        isDisconnecting = false;
      });

      this.connection!.input!.listen(_onDataReceived).onDone(() {
        if (isDisconnecting) {
          Utils.showSnackBar2("Device Disconnected!!");
        } else {
          Utils.showSnackBar2("Device Disconnected!!");
        }
        if (mounted) {
          setState(() {});
        }
      });
    }).catchError((error) {Utils.showSnackBar("Connection failed, please try again later");
    Navigator.push(
      context,
      MaterialPageRoute(builder: (context) => const BluetoothConnectionTask()),


    );
    showDialog(
      context: context,
      builder: (BuildContext context) {
        return AlertDialog(
          title: const Text('Error occurred while connecting'),
          content: const Text(
              "Connection failed. Please Try connecting to your device again"),
          actions: <Widget>[
            TextButton(
              child: const Text("Close",style: TextStyle(color: Colors.black),),
              onPressed: () {
                Navigator.of(context).pop();
              },
            ),
          ],
        );
      },
    );
    });
  }

Then, in the same class, I have another function (startDataCollection) that looks like this:

 void startDataCollection() async {
    startFlag = startFlag.trim();

    try {
      List<int> list = startFlag.codeUnits;
      Uint8List bytes = Uint8List.fromList(list);
      connection!.output.add(bytes);
      await connection!.output.allSent;
    } catch (e) {
      print(e);
    }
  }

At this point, there is no issue. However when I tried calling startDataCollection function from another screen (DataCollectionTimer Class), I got an error that

Null check operator used on a null value
which was referring to "connection" from the startDataCollection function.

Basically, I established a bluetooth connection with some hardware when I called the function establishConnection for the first time, and when I called it from the class DataCollectionTimer, it was still connected, but for some reason the connection var was empty when I called it from the DataCollectionPage class.

How can I use the function startDataCollection from another screen correctly?

If you need any more elaboration please let me know and thank you in advance.