fixed pending share race error on warm app start

lib/view/pages/files/files_upload_dialog.dart

@@ -187,11 +187,23 @@ class _FilesUploadDialogState extends State<FilesUploadDialog> {
             '${_uploadableFiles.indexOf(file) + 1}/${_uploadableFiles.length}';
       });
 
+      // A vanished source file would otherwise surface as a cryptic
+      // "Content-Length must contain only digits" HttpException, because
+      // statSync reports size -1 for missing files.
+      final fileStat = FileStat.statSync(filePath);
+      if (fileStat.type == FileSystemEntityType.notFound || fileStat.size < 0) {
+        _showUploadError(
+          'Die Datei "$fileName" ist nicht mehr verfügbar. '
+          'Bitte wähle sie erneut aus.',
+        );
+        return;
+      }
+
       final HttpClientResponse uploadTask;
       try {
         uploadTask = await webdavClient.putFile(
           File(filePath),
-          FileStat.statSync(filePath),
+          fileStat,
           PathUri.parse(fullRemotePath),
           onProgress: (progress) {
             setState(() {