fix: Handle null values in IconOverlayTypeString

Address a potential null reference issue by adding null checks when retrieving the "p.IconOverlay" value from the data store and providing a default value of IconOverlay.Liaher.

FoliCon/Models/Constants/GlobalVariables.cs

@@ -20,5 +20,5 @@ public static IconOverlay IconOverlayType()
     public const string MediaInfoFile = "info.folicon";
 
     private static string IconOverlayTypeString =>
-        Services.Tracker.Store.GetData("PosterIconConfigViewModel")["p.IconOverlay"].ToString();
+        Services.Tracker.Store.GetData("PosterIconConfigViewModel")?["p.IconOverlay"]?.ToString() ?? IconOverlay.Liaher.ToString();
 }