How can I fix "Future isn't a type" error in flutter?

I'm trying to store app data of my flutter application in the database, but the complier keeps showing "Future isn't a type" for async func and underlines in red. I've tried removing .analysis-driver as well, but that doesn't help. How can I fix it ?

Code:

import 'package:sqflite/sqflite.dart';
import 'package:path_provider/path_provider.dart';
import 'package:path/path.dart';
import 'dart:io';

class Db_Help{

  static final _db_name="Work_tasks.db";
  static final _db_version=1;
  static final table="Work Table";

  static final task_id="ID";
  static final task="Task";
  static final bool_check="Value";

  static Database _database;

  Db_Help._private_cons();
  static final Db_Help instance=Db_Help._private_cons();

  _initDatabase() async{
    Directory docu_dir=await getApplicationDocumentsDirectory();
    String path=join(docu_dir.path,_db_name);

    return await openDatabase(path, version: _db_version, onCreate: _onCreate);
  }

  Future<Database> get database async{

    if(_database!=null)
      return _database;

    _database=await _initDatabase();
    return _database;

  }

  Future _onCreate(Database db, int version) async{
    await db.execute('''
    CREATE TABLE $table (
    $task_id INTEGER PRIMARY KEY,
    $task TEXT NOT NULL,
    $bool_check INTEGER)
    ''');
  }

  Future<int> insert(Map<String,dynamic> row) async{
    Database db= await instance.database;
    return await db.insert(table, row);
  }

 }

I saw this problem last week. Here is what you can do, moving in that order if not fixed:

1. Make sure you have the dart:async import

2. Make sure your project level sdk is set correctly

File -> Project Structure In Project, select valid Project SDK. In Modules -> myapp_android -> Dependencies, select a valid Module SDK.

3. Update the SDK version in your pubspec.yaml

4. Flush your cache and restart Android Studio.

File -> Invalidate Caches/Restart and select "Invalidate Caches and Restart"