未定义命名参数“ child”。尝试将名称更正为现有命名参数的名称

我有一个简单的对话框窗口小部件，它在Beta通道上的Windows上没有错误的情况下也能正常工作，但是当我在MacBook上拉同一个存储库时，会出现错误。两种环境都在Beta频道上

未定义命名参数“ child”。尝试将名称更正为现有命名参数的名称，或定义名称为'child'的命名参数

我的对话。是什么导致孩子抱怨child: new AlertDialog(？

void _showDialog(
      BuildContext context, QRViewController controller, String result) async {
    await showDialog<String>(
      context: context,
      // FIXME: child isn't defined for this function. need to use right params
        child: new AlertDialog(
        contentPadding: const EdgeInsets.all(16.0),
        content: Container(
          height: 400,
          child: Column(
            children: [
              new Row(
                children: [
                  Text(
                    'Please record your body temperature \nin Celsius from the digital reading \non the infrared thermometer',
                  ),
                ],
              ),
              new Row(
                children: <Widget>[
                  new Expanded(
                    child: new TextField(
                      autofocus: true,
                      onChanged: (value) {
                        temp = value;
                      },
                      decoration: new InputDecoration(hintText: 'C'),
                      keyboardType: TextInputType.number,
                      inputFormatters: <TextInputFormatter>[
                        FilteringTextInputFormatter.allow(
                            RegExp(r'^\d+\.?\d*')),
                      ],
                    ),
                  )
                ],
              ),
              Row(
                children: [
                  Padding(
                    padding: const EdgeInsets.all(8.0),
                    child: Image.asset("assets/images/temp_checkin.jpg",
                        height: 250, fit: BoxFit.scaleDown),
                  ),
                ],
              )
            ],
          ),
        ),
        actions: <Widget>[
          new FlatButton(
              child: const Text('CANCEL'),
              onPressed: () {
                _QRViewExampleState.capturedQRCode = false;
                Navigator.pop(context);
              }),
          new FlatButton(
              child: const Text('SUBMIT'),
              onPressed: () {
                Navigator.pop(context);
                if (temp != '' && temp != null) {
                  _temperatureCheckIn(result, temp, context);
                }
                _QRViewExampleState.capturedQRCode = false;
              })
        ],
      ),
    );
  }