import random


def generate_new_foods(blocks):
    """If their is vacancy, randomly choose one; otherwise, return None"""
    
    # Get a list of vacant positions
    vacancy = [(i, j) for i in range(len(blocks)) for j in range(len(blocks[i])) if blocks[i][j] is None]
    
    # Empty
    if len(vacancy) == 0:
        return None
    else:
        return random.choice(vacancy)