分配字符串/匿名方法中的空引用

Question

当我在 onUpdateSlabPicture 函数中创建一个字符串类型变量时，我收到一个Null Reference异常（见下文）。

注意“ ApplicationManager.Istance.Paths.SlabPictureFile”在每个部分（Istance，Paths和SlabPictureFile）中都是不为空，我可以在手表中看到它们，而我可以将它们分配给变量。

这种情况：

// This method starts a task and when it's finished, it calls the other method below
private void takeAndCalibratePhoto()
{
    if(!_startExecutionTakeAndCalibratePhoto)
    {
        MiMessage viewMessage = new MiMessage();
        waitOperation(
            (token) =>
            {
                _startExecutionTakeAndCalibratePhoto = true;
                string errorMessage = "";
                if(!_CameraManager.TakeAndCalibratePhoto(viewMessage, token, out errorMessage))
                {
                    StopAction(null);
                }
                return errorMessage;
            },
            (p) =>
            {
                string errorMessage = (string)p;
                if(errorMessage.Length > 0)
                {
                    StopAction(null);
                    ApplicationManager.Istance.ShowInformationMessage(errorMessage);
                }
                else
                {
                    onUpdateSlabPicture();
                }
                _startExecutionTakeAndCalibratePhoto = false;
            },
            viewMessage, true);
    }
}

protected override void onUpdateSlabPicture()
{
    if (_CameraManager.Photo.SlabBitmap != null)
{
        // vvvvvvvv Null reference Exception vvvvvvvvvvvvvvvv
    string fileDaSalvare = ApplicationManager.Istance.Paths.SlabPictureFile;
        // ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        if (_Options.SelectionSlabImage)
        {
            List<string> listaNomi = new List<string>();
            ApplicationManager.Istance.ShowQuertyPad("Slab Name".Translate(), System.IO.Path.GetFileNameWithoutExtension(ApplicationManager.Istance.Paths.SlabPictureFile), listaNomi, (name) => { fileDaSalvare = name; });
        }
        _CameraManager.Photo.SlabBitmap.Save(fileDaSalvare);
        _CameraManager.Photo.SlabBitmap.Dispose();
        _CameraManager.Photo.SlabBitmap = null;
    }
}

为什么？此外，还有一个不太重要的问题：为什么我要用这种方式编写代码：

if (_CameraManager.Photo.SlabBitmap != null)
{
    string fileDaSalvare;

        if (_Options.SelectionSlabImage)
        {
            List<string> listaNomi = new List<string>();
            ApplicationManager.Istance.ShowQuertyPad("Slab Name".Translate(), System.IO.Path.GetFileNameWithoutExtension(ApplicationManager.Istance.Paths.SlabPictureFile), listaNomi, (name) => { fileDaSalvare = name; });
        }
        else
        {
            fileDaSalvare = ApplicationManager.Istance.Paths.SlabPictureFile;
        }
        _CameraManager.Photo.SlabBitmap.Save(fileDaSalvare);
    }
}

编译器无法识别匿名方法中字符串的初始化吗？