getting an error the argument type 'Object?' can not b assigned to 'Object' in flutter

I am creating a demo of hive+getx and here I want to initilize transactions from box’s values

but I am getting an error
The Argument Type ‘TransactionModel?’ can’t be assign to the parameter type ‘TransactionModel’

here is my coding

class TransactionController extends GetxController
{
  List<TransactionModel> _transactions=[];
  Box<TransactionModel> transactionbox=Hive.box('transactionsdb');

  List<TransactionModel> get transactions{
    return _transactions;
  }

  TransactionController(){
    for(int x=0;x<transactionbox.values.length;x++)
      {
        _transactions.add(transactionbox.getAt(x));
//this statement showing error line
      }


  }



  int get countlength{
    return _transactions.length;
  }

  void addTransaction(TransactionModel transaction)
  {
    _transactions.add(transaction);
    update();
  }

  void deleteTransaction(String id)
  {
    _transactions.removeWhere((element) => element.id==id);
    update();
  }

  // void swapTransaction(TransactionModel transaction)
  // {
  //   transaction.isExpense=!transaction.isExpense;
  //   update();
  // }

  void swapTransaction(TransactionModel transaction)
  {
    int index=_transactions.indexOf(transaction);
    _transactions[index].isExpense=!_transactions[index].isExpense;
    update();
  }


}

>Solution :

You must check if the value of transactionbox.getAt(x) is null or not befor add it to the _transactions list.

  TransactionController(){
    for(int x=0;x<transactionbox.values.length;x++) {
      var valueToCheck = transactionbox.getAt(x);
      if (valueToCheck != null) {
        _transactions.add(valueToCheck);
      }
    }
  }