Fix export dialog: single selection mode, null-safe set lookup in o8c export

Author: kellyelton

octgnFX/Octgn.JodsEngine/DeckBuilder/ExportCardImagesDialog.xaml

@@ -50,7 +50,7 @@
             <!-- Card List -->
             <ListBox Grid.Row="2"
                      x:Name="CardImagesList"
-                     SelectionMode="Extended"
+                     SelectionMode="Single"
                      Margin="0,0,0,8"
                      ScrollViewer.HorizontalScrollBarVisibility="Disabled"
                      PreviewKeyDown="CardImagesList_PreviewKeyDown">

octgnFX/Octgn.JodsEngine/DeckBuilder/ExportCardImagesDialog.xaml.cs

@@ -219,9 +219,9 @@ private void ExportToO8c(List<CardImageItem> cards, string filename)
             {
                 foreach (var card in cards)
                 {
-                    var set = card.Card.GetSet();
+                    var setId = card.Card.GetSet()?.Id ?? card.Card.SetId;
                     var imageFileName = Path.GetFileName(card.ImagePath);
-                    var entryName = $"{_game.Id}/Sets/{set.Id}/Cards/{imageFileName}";
+                    var entryName = $"{_game.Id}/Sets/{setId}/Cards/{imageFileName}";
 
                     archive.CreateEntryFromFile(card.ImagePath, entryName, CompressionLevel.Optimal);
                 }