Error: The method 'execute' isn't defined for the class 'Type' - flutter

Question

The method 'execute' isn't defined for the type 'Type'
How can I solve the error I get above?
import 'package:sqflite/sqflite.dart';
import 'package:path/path.dart';
class LiteDb {
static Database? _db;
Future<Database?> get getInstance async {
if (_db == null) {
_db = await instance();
return _db;
else {
return _db;
}
}
Future<void> addSalaryColumn() async {
final db = await Database;
await db.execute(
"ALTER TABLE PERSON ADD COLUMN salary INTEGER DEFAULT 0"
);
}
instance() async {
Get a location using getDatabasesPath
var databasesPath = await getDatabasesPath();
String path = join(databasesPath, 'lite_sql.db');
open the database
Database database = await openDatabase(path, version: 2,
onCreate: (Database db, int version) async {
When creating the db, create the table
await db.execute(
CREATE TABLE PERSON (id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT NOT NULL, age INTEGER);
CREATE TABLE ACCOUNT (id INTEGER PRIMARY KEY AUTOINCREMENT, PERSON_ID INTEGER NOT NULL, ACCOUNT INTEGER NOT NULL, VALUE REAL);
''');
print('Text Database has been created');
},
onUpgrade: (Database db, int oldVersion, int newVersion) async {
if (newVersion >= 2) {
await db.execute('''
ALTER TABLE PERSON ADD COLUMN salary REAL NULL
''');
}
}
);
print(' Database connected');
return database;
}
inquiry(String sqlTxt) async {
Database? db = await getInstance;
Get the records
List<Map> list = await db!.rawQuery(sqlTxt);
return list;
}
insert(String sqlTxt) async {
Database? db = await getInstance;
Insert some record
int count = await db!.rawInsert(sqlTxt);
return count;
}
update(String sqlTxt) async {
Database? db = await getInstance;
Update some record
int count = await db!.rawUpdate(sqlTxt);
return count;
}
delete(String sqlTxt) async {
Database? db = await getInstance;
Delete some record
int count = await db!.rawDelete(sqlTxt);
return count;
}
}