fix:Index out of bounds.

src/Ursa/Controls/PathPicker/PathPicker.cs

@@ -139,7 +139,7 @@ protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs chang
         {
             _twoConvertLock = true;
             var stringBuilder = new StringBuilder();
-            stringBuilder.Append(SelectedPaths[0]);
+            stringBuilder.Append(SelectedPaths.Count != 0 ? SelectedPaths[0] : string.Empty);
             foreach (var item in SelectedPaths.Skip(1))
             {
                 stringBuilder.AppendLine(item);