请问这个是什么原因，谢谢！Traceback (most recent call last):
File "/Users/kanghuayu/Library/Application Support/Sublime Text 3/Packages/User/Alien_invasion/alien_invasion.py", line 24, in <module>
run_game()
File "/Users/kanghuayu/Library/Application Support/Sublime Text 3/Packages/User/Alien_invasion/alien_invasion.py", line 12, in run_game
screen = pygame.display.set_mode((ai_settings.screen_width,ai_settings.screen_height))
AttributeError: 'Settings' object has no attribute 'screen_width'