Type 'null' is not a subtype of type 'string' while trying to retrieve data from list into listview

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Dart

Flutter

This problem is driving me crazy and I don't know what to do anymore I'm completely lost and it's ruining my life.
I'm trying to insert a list values which I retrieved from MySQL database into listView when the user pressed a button but the screen throws

type 'Null' is not a subtype of type 'String' 

instead

This is how the values list was added

bool _isFromLogin = false;
List<Uint8List?> _imageByteValues = <Uint8List>[];
List<String?> _fileValues = <String>[];
List<File?> _imageValues = <File>[];
List<Icon?> _nonImgValues = <Icon>[];
String _FileType = "";
String _getTable = "";

class cakeLogin extends StatefulWidget {
  @override
  cakeLoginPageMain createState() => cakeLoginPageMain(); 
}

class cakeLoginPageMain extends State<cakeLogin> {

  String? _CustEmailInit; 
  BuildContext? loginContext;

  List<String?> _fileNameStores = <String>[];
  List<Uint8List?> _imgByteStores = <Uint8List>[];
  List<String?> _dateRetrieveStores = <String>[];

  Future<void> removeOldData() async {
    setState(() {
      _fileNameStores.clear();
    });

    setState(() {
      _imgByteStores.clear();
    });

    setState(() {
      _fileValues.clear();
    });

    setState(() {
      _imageValues.clear();
    });

    setState(() {
      _nonImgValues.clear();    
    });

    setState(() {
      _imageByteValues.clear();
    });
  }

  Future<void> callData() async {

    await removeOldData();

    var custUsernameGet = await UsernameGetter().Connection(_CustEmailInit!);
    var nameValuesGet = await NameGetter().Connection(custUsernameGet);

    setState(() {
      _custUsername = custUsernameGet;
    });

    // ----- File name retrieval starts -----

    setState(() {
      _fileNameStores.addAll(nameValuesGet);
    });
    setState(() {
      _fileValues.addAll(_fileNameStores.toSet().toList());
    });

    // ----- File name retrieval end -----

    // Start retrieving leading image

    var retrieveCustFile = await LoginGetter().connectionFunc(custUsernameGet, "image_info");
    setState(() {
      _imgByteStores.addAll(retrieveCustFile);
    });

    setState(() {
      _imageByteValues.addAll(_imgByteStores.toSet());
    });

    // Retrieving leading image end
    
    var retrieveUploadDate = await DateGetter().connectionFunc(custUsernameGet,"image_info");
    setState(() {
      _dateStoresValues.addAll(retrieveUploadDate);
    });
}
}

NameGetter class (retrieve file names from database)

class NameGetter {

  Future<List<String>> Connection(String _custUsername) async {
    final conn = await MySQLConnection.createConnection(
      
    );
    await conn.connect();

    var _getFileNamesQue = await conn.execute("SELECT FILE_NAME FROM " + "image_info" + " WHERE CUST_USERNAME = :username",
      {
        "username": _custUsername
      });

    List<String>? _FileNameValues = <String>[]; 
    for(var _rowsOfFileNames in _getFileNamesQue.rows) {
      var _retrieveNames = await _rowsOfFileNames.assoc()['FILE_ANME']!;
      _FileNameValues.add(_retrieveNames);
    }

    return await _FileNameValues;
  }
}

UsernameGetter class (Retrieve user username from their entered email)

class UsernameGetter {
  Future<String> Connection(String _custEmail) async {
    final conn = await MySQLConnection.createConnection(

    );

    await conn.connect();

    var _getUsernameQue = await conn.execute("SELECT CUST_USERNAME, CUST_EMAIL FROM information WHERE CUST_EMAIL = :email", 
    {
      "email": _custEmail
    });

    String? _usernameDetected;
    for(final _rowsOfEmail in _getUsernameQue.rows) {
      _usernameDetected = _rowsOfEmail.assoc()['CUST_USERNAME']!;
    }
    return await _usernameDetected!;
  }
}

Initialize widget function for listView. And this is where the problem start, the list _fileValues supposed to have length of 2 but when I print the length from this Widget function the value returns 0 instead of 2.

Widget setupFileList(BuildContext context) {
  var  mainReturn;
  var _showList = SingleChildScrollView(
        child: Column(
          children: [
            ListView.separated(
              shrinkWrap: true,
              separatorBuilder: (BuildContext context, int index) => Divider(height: 1,color: Color.fromARGB(255,48, 48, 48)),
              itemBuilder: (BuildContext _context, int _index) {
              return GestureDetector(
                child: Container(              
                  color: setupMainTheme,
                  child: ListTile(
                    onTap: () {

                      FilePositon = _index;
                      var setTitle = _fileValues[FilePositon];
                      SelectedFileName = setTitle;
                      var setFileType = setTitle!.substring(setTitle.length-3);
                      _FileType = setFileType;

                    },

                    leading: SizedBox(
                      child: _isFromLogin == false ? Image.file(_imageValues[_index]!) : Image.memory(_imageByteValues[_index]!),scale: .8),
                      height: 48,
                      width: 48
                    ), 
                    trailing: Icon(Icons.arrow_right,color: Colors.grey),
                    title: Text(Text(_fileValues[_index]!.toString(),
                      style: TextStyle(
                        color: Color.fromARGB(213, 255, 255, 255),
                      )),
                    subtitle: Text(Text(_isFromLogin == false ? _dateToStr : _dateStoresValues[_index]!.toString(), 
                      style: TextStyle(
                        color: Color.fromARGB(197, 255, 255, 255),
                        fontSize: 12.8)),
                  )
                )
              );
              },

            itemCount: _fileValues.length,
            
            ),
          ],
        ),
    ); 

  print(_fileValues.length);
  if(_fileValues.length > 0) {
    mainReturn = _showList;
  } else {
    mainReturn = redundaneShow();
  }
  return mainReturn; // type null is not a subtype of string
}

Call and display the listView widget function (setupFileList) into the screen

class _cakeHomeWidgetState extends State<cakeWidget> { //State<cakeWidget>

  BuildContext? dialogContext;
      
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: setupMainTheme,
      appBar: PreferredSize(
        preferredSize: Size.fromHeight(58),
        child: AppBar(
          centerTitle: false,
          title: Column(
            children: <Widget>[
             Text("Welcome",
                style: 
                  TextStyle(
                    color: Color.fromARGB(255, 190, 190, 190),
                    fontWeight: FontWeight.w700,
                    fontSize: 20
                    )
                ),
            ], 
          ),
          automaticallyImplyLeading: false,
          backgroundColor: setupMainTheme,
          shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.vertical(
              bottom: Radius.circular(12)),
            ),
        ),
      ),

    body: setupFileList(context),

What I have tried:

I'm a bad programmer, I made a lot of mistakes and I don't know how to fix them. Reading documentations does not helps, google does not help, I just want to fix this bug and quit.