Fix WPF preview not working

Author: IruzzArcana

UndertaleModTool/Controls/System/TabControlDark.cs

@@ -18,7 +18,7 @@ public TabControlDark()
 
         private void TabControlDark_Loaded(object sender, RoutedEventArgs e)
         {
-            SetDarkMode(Settings.Instance.EnableDarkMode);
+            SetDarkMode(Settings.Instance is not null ? Settings.Instance.EnableDarkMode : false);
         }
 
         /// <inheritdoc/>

UndertaleModTool/Converters/IsVersionAtLeastConverter.cs

@@ -16,7 +16,8 @@ public class IsVersionAtLeastConverter : IValueConverter
 
         public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
         {
-            if (mainWindow.Data?.GeneralInfo is null
+            if (mainWindow is null
+                || mainWindow.Data?.GeneralInfo is null
                 || parameter is not string verStr
                 || verStr.Length == 0)
                 return Visibility.Hidden;