private async void FileControl_Loaded(object sender, RoutedEventArgs e)
        {
            FileControl control = (FileControl)sender;

            AddFileControl(control);

            if (isFirstOpening)
            {
                await SetCurrentContent();
            }
        }
        private void AddFileControl(FileControl control)
        {
            if (fileControls.Contains(control))
            {
                return;
            }

            control.Controls = new CustomViewerControls(cbrBottom, abbStop, Frame);
            control.Api      = viewing.Api;

            fileControls.Add(control);
        }