Fixes problems with windows:

- Changed cPlayer:OpenWindow to accept a ref, tolua adds a nil check
- Close open lua window in destructor, to avoid dangling pointers

src/Entities/Minecart.cpp

@@ -1222,7 +1222,7 @@ void cMinecartWithChest::OnRightClicked(cPlayer & a_Player)
 	{
 		if (a_Player.GetWindow() != Window)
 		{
-			a_Player.OpenWindow(Window);
+			a_Player.OpenWindow(*Window);
 		}
 	}
 }