fix(Core/Map): Ensure grid is valid before unloading to prevent potential … (#24621)

Co-authored-by: Roman BADANIN <r.badanin@defi-informatique.fr>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/server/game/Maps/Map.cpp

@@ -1037,9 +1037,10 @@ void Map::UnloadAll()
 
     for (GridRefMgr<MapGridType>::iterator i = GridRefMgr<MapGridType>::begin(); i != GridRefMgr<MapGridType>::end();)
     {
-        MapGridType& grid(*i->GetSource());
+        MapGridType* grid = i->GetSource();
         ++i;
-        UnloadGrid(grid); // deletes the grid and removes it from the GridRefMgr
+        if (grid)
+            UnloadGrid(*grid);
     }
 
     // pussywizard: crashfix, some npc can be left on transport (not a default passenger)