The method was called on null even though method was already declared - loops

Question

I was making an app and it keeps showing an error the method was called on null. When I looked up for answers it was specified that this occurs when method is not declared but in my case method has been already declared.
Due to this error I am not able to save data in my app.
initialiseDatabase is the function it states as called on null.
Here is the code-
static DatabaseHelper _databaseHelper; // singleton
static Database _database;
String noteTable = 'note_Table';
String colId = 'id';
String colTitle = 'title';
String colDescription = 'description';
String colPriority = 'priority';
String colDate = 'date';
DatabaseHelper._createInstance();// named constructor to create dbms helper
factory DatabaseHelper(){
if(DatabaseHelper == null) {
_databaseHelper = DatabaseHelper._createInstance();
}
return _databaseHelper;
}
Future<Database> get database async{
if(_database == null){
_database = await initializeDatabase();
}
return _database;
}
Future<Database> initializeDatabase() async {
Directory directory = await getApplicationDocumentsDirectory();
String path = directory.path + 'notes.db';
var notesDatabase = await openDatabase(path, version: 1, onCreate: _createDb);
return notesDatabase;
}
void _createDb(Database db, int newVersion) async{
await db.execute('CREATE TABLE $noteTable($colId INTEGER PRIMARY KEY AUTOINCREMENT, $colTitle TEXT.'
'$colDescription TEXT, $colPriority INTEGER, $colDate TEXT');
}
// Fetch Operation: Get all note objects from database
Future<List<Map<String, dynamic>>> getNoteMapList() async {
Database db = await this.database;
// var result = await db.rawQuery('SELECT * FROM $noteTable order by $colPriority ASC');
var result = await db.query(noteTable, orderBy: '$colPriority ASC');
return result;
}
// Insert Operation: Insert a Note object to database
Future<int> insertNote(Note note) async {
Database db = await this.database;
var result = await db.insert(noteTable, note.toMap());
return result;
}
// Update Operation: Update a Note object and save it to database
Future<int> updateNote(Note note) async {
var db = await this.database;
var result = await db.update(noteTable, note.toMap(), where: '$colId = ?', whereArgs: [note.id]);
return result;
}
// Delete Operation: Delete a Note object from database
Future<int> deleteNote(int id) async {
var db = await this.database;
int result = await db.rawDelete('DELETE FROM $noteTable WHERE $colId = $id');
return result;
}
// Get number of Note objects in database
Future<int> getCount() async {
Database db = await this.database;
List<Map<String, dynamic>> x = await db.rawQuery('SELECT COUNT (*) from $noteTable');
int result = Sqflite.firstIntValue(x);
return result;
}
// Get the 'Map List' [ List<Map> ] and convert it to 'Note List' [ List<Note> ]
Future<List<Note>> getNoteList() async {
var noteMapList = await getNoteMapList(); // Get 'Map List' from database
int count = noteMapList.length; // Count the number of map entries in db table
List<Note> noteList = List<Note>();
// For loop to create a 'Note List' from a 'Map List'
for (int i = 0; i < count; i++) {
noteList.add(Note.fromMapObject(noteMapList[i]));
}
return noteList;
}
}
While this is the function which it states as not declared
Future<Database> get database async{
if(_database == null){
_database = await initializeDatabase();
}
return _database;
}
Future<Database> initializeDatabase() async {
Directory directory = await getApplicationDocumentsDirectory();
String path = directory.path + 'notes.db';
var notesDatabase = await openDatabase(path, version: 1, onCreate: _createDb);
return notesDatabase;
}