Flutter Bluetooth Null Check Exception although null was checked - flutter

Question

Here is the main() function. Here is the initializisation of the "flutter_isolate" library
void main() {
WidgetsFlutterBinding.ensureInitialized();
runApp(const MyApp());
}
...
class BTConnection {
BluetoothConnection? c;
Future<void> connect(address) async {
print('Connecting to the device ...');
BluetoothConnection connection =
await BluetoothConnection.toAddress(address);
c = connection;
print('Connected to the device');
FlutterIsolate.spawn(listenToData, connection);
}
Future<void> disconnect() async {
if (c != null) {
await c?.finish();
print("disconnected from device");
} else {
print("There is no bluetooth connection to disconnect");
}
}
#pragma('vm:entry-point')
Future<void> listenToData(connection) async {
print("listening ...");
await connection.input?.listen((Uint8List data) {
print('Data incoming: ${ascii.decode(data)}');
connection.output.add(data); // Sending data
}).onDone(() {
print('Disconnected by remote request');
});
}
}
Whenever I run the code and enter the correct MAC address in the text field, the function connect() is called and after a few seconds the message "Connected to the device" is printed. The following error message is then output:
E/flutter (26066): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Null check operator used on a null value
E/flutter (26066): #0 FlutterIsolate.spawn
flutter_isolate.dart:28
E/flutter (26066): #1 BTConnection.connect
main.dart:110
E/flutter (26066): <asynchronous suspension>
E/flutter (26066): #2 _HomePageState.build.<anonymous closure>
main.dart:70
E/flutter (26066): <asynchronous suspension>
E/flutter (26066):