[Python] Pygame in python

My game was working before I added lines 7 and 8. what happened in the game was the red sprites that fall down the screen would hit the main character sprite which is green and the game would stop just as I need it to.

import pygame
import random

from os import path
img_dir = path.join(path.dirname(__file__),'PNG')

background = pygame.image.load(path.join(img_dir,'Space-Background.png')).convert()
background_rect = background.get_rect()

WIDTH = 480
HEIGHT = 600
FPS = 60

WHITE = (255,255,255)
BLACK = (0,0,0)
RED = (255,0,0)
GREEN = (0,255,0)
BLUE = (0,0,255)


pygame.init()
pygame.mixer.init()

screen = pygame.display.set_mode((WIDTH,HEIGHT))
pygame.display.set_caption("My Game")
clock = pygame.time.Clock()

class Player(pygame.sprite.Sprite):

def __init__(self):

pygame.sprite.Sprite.__init__(self)
self.image = pygame.Surface((50,40))
self.image.fill(GREEN)
self.rect = self.image.get_rect()
self.rect.centerx = WIDTH/2
self.rect.bottom = HEIGHT-10

self.speedx = 0

def update(self):
self.speedx = 0
keystate = pygame.key.get_pressed()
if keystate[pygame.K_LEFT]:
self.speedx = -5
if keystate[pygame.K_RIGHT]:
self.speedx = 5

self.rect.x += self.speedx

if self.rect.right > WIDTH:
self.rect.right = WIDTH
if self.rect.left < 0:
self.rect.left = 0

def shoot(self):
bullet = Bullet(self.rect.centrex,self.rect.top)
all_sprites.add(bullet)
bullets.add(bullet)

class Mob(pygame.sprite.Sprite):
def __init__(self):
pygame.sprite.Sprite.__init__(self)
self.image = pygame.Surface((30,40))
self.image.fill(RED)
self.rect = self.image.get_rect()

self.rect.x = random.randrange(0, WIDTH - self.rect.width)
self.rect.y = random.randrange(-100,-40)
self.speedy = random.randrange(1,8)

def update(self):
self.rect.y += self.speedy

if self.rect.top > HEIGHT +10:
self.rect.x = random.randrange(0, WIDTH - self.rect.width)
self.rect.y = random.randrange(-100,-40)
self.speedy = random.randrange(1,8)

class Bullet(pygame.sprite.Sprite):
def __init__(self,x,y):
pygame.sprite.Sprite.__init__(self)
self.image = pygame.Surface((10,20))
self.image.fill(YELLOW)
self.rect = self.image.get_rect()
self.rect.bottom = y
self.rect.centrex = x
self.speedy = -10
def update(self):
self.rect.y += self.speedy

if self.rect.bottom < 0:
self.kill()

# player_img = pygame.image.load(path.join(img_dir,"spaceShips_003.png")).convert()
# bullet_img = pygame.image.load(path.join(img_dir,"spaceMissile_006.png")).convert()
# mob_img = pygame.image.load(path.join(img_dir,"SpaceMeteor_004.png")).convert()

all_sprites = pygame.sprite.Group()
mobs = pygame.sprite.Group()
bullets = pygame.sprite.Group()

player = Player()

for i in range(8):
m = Mob()
all_sprites.add(m)
mobs.add(m)

all_sprites.add(player)

running = True
while running:
clock.tick(FPS)
for event in pygame.event.get():
if event.type == pygame.QUIT:
running = False

elif event.type == pygame.KEYDOWN:
if event.key == pygame.K_SPACE:
player.shoot()
all_sprites.update()

hits = pygame.sprite.groupcollide(mobs,bullets,True,True)
for hit in hits:
m = Mob()
all_sprites.add(m)

hits = pygame.sprite.spritecollide(player,mobs,False)

if hits:
running = False

screen.fill(BLACK)
# screen.split(background.background_rect)
all_sprites.draw(screen)
pygame.display.flip()


pygame.quit()


After I added the two lines for the images there was an error message. I am not sure what happened and I need some help fixing it.

>>> %Run trial.py
pygame 2.1.2 (SDL 2.0.18, Python 3.7.9)
Hello from the pygame community. https://www.pygame.org/contribute.html
Traceback (most recent call last):
File "C:\Users\Mish Mash\Documents\School File\trial.py", line 7, in <module>
background = pygame.image.load(path.join(img_dir,'Space-Background.png')).convert()
pygame.error: cannot convert without pygame.display initialized

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