你应该做更多的检查。这个问题是从DirectoryInfo模型开始的。正如你提到的，你必须检查路径是否不为空，而且你还需要检查目录是否真的存在。

string path = "";
        if (string.IsNullOrWhiteSpace(path))
        {
            //do something
        }

        DirectoryInfo directory = new DirectoryInfo(path);
        if (!directory.Exists)
        {
            //do something
        }

下面是如何在C#中处理异常：https://www.c-sharpcorner.com/article/csharp-try-catch/
在您的情况下，可以使用FirstOrDefault（）代替First，这样就可以返回null
这里只为你：

private void pictureBox1_DoubleClick(object sender, EventArgs e)
        {
            try
            {
                string rootPath = Environment.ExpandEnvironmentVariables(@"d:\downloads\main");

                DirectoryInfo? directory = new DirectoryInfo(rootPath).GetDirectories()
                    .OrderByDescending(d => d.CreationTimeUtc)
                    .FirstOrDefault();

                if (directory is null)
                {
                    if (string.IsNullOrEmpty(rootPath))
                        throw new Exception("Root path was empty.");
                    throw new DirectoryNotFoundException("Directory not found.");
                }

                Editor editor = new Editor();
                editor.Show();
            }catch(DirectoryNotFoundException dirEx)
            {
                MessageBox.Show(dirEx.Message);
                return;
            }catch(Exception ex)
            {
                MessageBox.Show(ex.Message);
                return;
            }
        }