[fix:] spplus crashed when not selecting any file

MainWindow.axaml.cs

@@ -90,6 +90,7 @@ public partial class MainWindow : Window
         });
 
         if (file == null) return;
+        if (file.Count == 0) return;
 
         var imported_students = import.ImportStudentsFromFile(file[0].Path.LocalPath.ToString());
         foreach (var s in imported_students)