未处理的异常:NoSuchMethodError:方法“call”是在null上调用的

我升级了颤振sdk flutter upgrade --force 。升级前项目运行良好,但升级后出现错误。我目前正在使用flutter的最新稳定版本。

E/flutter (17687): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: NoSuchMethodError: The method 'call' was called on null.
E/flutter (17687): Receiver: null
E/flutter (17687): Tried calling: call()
E/flutter (17687): #0      Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
E/flutter (17687): #1      BasicLock.synchronized (package:synchronized/src/basic_lock.dart:31:24)
E/flutter (17687): #2      SembastDatabase.flush (package:sembast/src/database_impl.dart:593:27)
E/flutter (17687): #3      SembastDatabase.open (package:sembast/src/database_impl.dart:879:11)
E/flutter (17687): <asynchronous suspension>
E/flutter (17687): #4      DatabaseOpenHelper.openDatabase.<anonymous closure> (package:sembast/src/database_factory_mixin.dart:80:7)
E/flutter (17687): <asynchronous suspension>
E/flutter (17687): #5      BasicLock.synchronized (package:synchronized/src/basic_lock.dart:33:16)
E/flutter (17687): <asynchronous suspension>
E/flutter (17687): #6      DaoDatabase._openDatabase (package:kitabyatra/db/DaoDatabase.dart:43:26)
E/flutter (17687): <asynchronous suspension>

我已实施如下:

class DaoDatabase {

  DaoDatabase();

 
  static final DaoDatabase _singleton = DaoDatabase();

  static DaoDatabase get instance => _singleton;

 
  Completer<Database> _dbOpenCompleter;


  Future<Database> get database async {
   
    if (_dbOpenCompleter == null) {
      _dbOpenCompleter = Completer<Database>();
      
      _openDatabase();
    }

    return _dbOpenCompleter.future;
  }

  Future<dynamic> _openDatabase() async {
    
    final Directory appDocumentDir = await getApplicationDocumentsDirectory();
  
    final String dbPath = join(appDocumentDir.path, Strings.appDbName);

    final Database database = await databaseFactoryIo.openDatabase(dbPath);  //--> Getting error here
   
    _dbOpenCompleter.complete(database);
  }
}