don't allow tiles with an ID of 0

src/game.rs

@@ -692,7 +692,7 @@ impl Game {
     /// adds a tile safely and returns the ID
     #[inline]
     pub fn add_tile(&mut self, mut tile: Tile) -> String {
-        if self.tile_ids().contains(&tile.id) {
+        if tile.id == "0".to_string() || self.tile_ids().contains(&tile.id) {
             let new_id = self.new_tile_id();
             if new_id != tile.id {
                 tile.id = new_id;