Why is the function not invoke - flutter

Question

I am new to coding and to flutter but am attempting to build an app for scanning, creating database, and inventory. I have been attempting to use flutter_barcode_scanner to achieve this along with several others while looking at multiple videos and blogs but have become unsuccessful and am now I'm coming here for help.
Provided is the code that I'm currently attempting to write for it. At this point I've confused myself since I was recently trying to utilize scan_preview also as my goal is to implement the scanner as a stream in half or 1/3rd of the screen and having the rest of the screen as the result that is pulled from the database or to create a new item if no match.
I am looking for some assistance in how to implement the barcode scanner in the fashion where it streams in part of the screen and why I am getting these issues and how to fix them. I have attempted the fixes that androidstudio is providing me but it creates many more issues.
Am I going about this the correct way?
How do I use flutter_barcode_scanner to stream in only a portion of the screen?
Errors from AndroidStudio:
child: ScanMode.BARCODE( Error: The expression doesn't evaluate to a function, so it can't be invoked
Center(child: Text(barcode)) Error: Undefined name 'barcode'
'#ff6666', 'Cancel', true, ScanMode.BARCODE)! Error: This requires the 'non-nullable' language feature to be enabled
return Scaffold(
appBar: AppBar(
title: Text("Barcode Scanner"),
centerTitle: true,
),
body: Column(
children: [
SizedBox(
width: double.infinity,
height: height / 0.6,
child: ScanMode.BARCODE(
onScanResult: (result) {
debugPrint('scan result: $result');
setState(() {
_data = result;
});
},
),
),
SizedBox(height: 20),
Center(child: Text(barcode))
],
));
}
}
class ScannerHelper extends StatefulWidget {
#override
_ScannerHelperState createState() => _ScannerHelperState();
}
class _ScannerHelperState extends State<ScannerHelper> {
String _scanbarcode = 'Unknown';
#override
void initState() {
super.initState();
}
Future<void> startBarcodeScanStream() async {
FlutterBarcodeScanner.getBarcodeStreamReceiver(
'#ff6666', 'Cancel', true, ScanMode.BARCODE)!
.listen((barcode) => print(barcode));
}
//build - to resolve scanner issue
#override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(title: const Text('Barcode scan')),
body: Builder(builder: (BuildContext context) {
return Container(
alignment: Alignment.center,
child: Flex(
direction: Axis.vertical,
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
ElevatedButton(
onPressed: () => startBarcodeScanStream(),
child: Text('Start barcode scan stream')),
Text('Scan result : $_scanbarcode\n',
style: TextStyle(fontSize: 20))
]));
})));
}