Fix crash when showing chapter panel for vanilla Prologue and Epilogue

Author: maddie480

UI/InGameOverworldHelper.cs

@@ -649,7 +649,7 @@ private static IEnumerator UpdateIconRoutine(OuiChapterPanel panel, OuiChapterSe
 
         public static void OpenChapterPanel(Player player, string sid, ChapterPanelTrigger.ReturnToLobbyMode returnToLobbyMode, bool savingAllowed) {
             AreaData areaData = (AreaData.Get(sid) ?? AreaData.Get(0));
-            if (!Dialog.Has(areaData.Name + "_collabcredits") && areaData.Mode[0].Checkpoints.Length > 0) {
+            if (!Dialog.Has(areaData.Name + "_collabcredits") && areaData.Mode[0].Checkpoints?.Length > 0) {
                 // saving isn't compatible with checkpoints, because both would appear on the same page.
                 savingAllowed = false;
             }