Fix concurrent access to entity list issue

dungeonbattle/entities/items.py

@@ -94,7 +94,7 @@ class Bomb(Item):
         Special exploding action of the bomb
         """
         if self.exploding:
-            for e in m.entities:
+            for e in m.entities.copy():
                 if abs(e.x - self.x) + abs(e.y - self.y) <= 1 and \
                         isinstance(e, FightingEntity):
                     e.take_damage(self, self.damage)