flutter - The getter 'preferredSize' isn't defined for the type 'Widget'

Question

@override
  Widget build(BuildContext context) {
    final mediaQuery = MediaQuery.of(context);
    final isLandscape = mediaQuery.orientation == Orientation.landscape;
    final appBar = Platform.isIOS
        ? CupertinoNavigationBar(
            middle: Text(
              'Personal Expenses',
            ),
            trailing: Row(
              mainAxisSize: MainAxisSize.min,
              children: <Widget>[
                GestureDetector(
                  child: Icon(CupertinoIcons.add),
                  onTap: () => _startAddNewTransaction(context),
                ),
              ],
            ),
          )
        : AppBar(
            title: Text(
              'Personal Expenses',
            ),
            actions: <Widget>[
              IconButton(
                icon: Icon(Icons.add),
                onPressed: () => _startAddNewTransaction(context),
              ),
            ],
          );

The getter 'preferredSize' isn't defined for the type 'Widget'.

    final txListWidget = Container(
      height: (mediaQuery.size.height -
              appBar.preferredSize.height -
              mediaQuery.padding.top) *
          0.7,

The argument type 'Widget' can't be assigned to the parameter type 'ObstructingPreferredSizeWidget?'

 return Platform.isIOS
    ? CupertinoPageScaffold(
        child: pageBody,
        navigationBar: appBar,
      )

The argument type 'Widget' can't be assigned to the parameter type 'PreferredSizeWidget?'

: Scaffold(
        appBar: appBar,
        body: pageBody,

Answer 1

Please use data type for the variable appBar

final PreferredSizeWidge appBar = Platform.isIOS ? ......