避免下载时的可能出现的问题

DownKyi/Services/Download/AddToDownloadService.cs

@@ -185,7 +185,7 @@ namespace DownKyi.Services.Download
             // 下载设置dialog中如果点击取消或者关闭窗口，
             // 会返回空字符串，
             // 这时直接退出
-            if (directory == string.Empty) { return null; }
+            if (directory == null || directory == string.Empty) { return null; }
 
             // 文件夹不存在则创建
             if (!Directory.Exists(directory))

DownKyi/ViewModels/Dialogs/ViewDownloadSetterViewModel.cs

@@ -56,7 +56,7 @@ namespace DownKyi.ViewModels.Dialogs
             {
                 SetProperty(ref directory, value);
 
-                if (directory != string.Empty)
+                if (directory != null && directory != string.Empty)
                 {
                     DriveName = directory.Substring(0, 1).ToUpper();
                     DriveNameFreeSpace = Format.FormatFileSize(HardDisk.GetHardDiskFreeSpace(DriveName));
@@ -381,7 +381,7 @@ namespace DownKyi.ViewModels.Dialogs
 
             // 弹出选择下载目录的窗口
             path = DialogUtils.SetDownloadDirectory();
-            if (path == null || path == "")
+            if (path == null || path == string.Empty)
             {
                 return null;
             }