AS3：TypeError：错误＃1009：无法访问空对象引用的属性或方法

I've been working on a flash game for an assignment and it's been a real headache so far. The game's relatively simple, collect items (mementos), avoid enemies or lose health (sanity), collect all items on the screen to level up (items, player and enemies reset, score remains the same), get hit 5 times and you get the game over screen.I've been working on a flash game for an assign