Function "doesn't recognise" a python class method

I have written a class that creates a board for a Klotski game and I want to use breadth-first search to solve the given problem. Here is a bit of my code:

EDIT and DISCLAIMER: I added all the relevant class methods and functions in the code, although it is not optimized and I tried to make it shorter...

from copy import deepcopy

class Klotski:

    def __init__(self, board, history=[]):

        #Initialize the board as a matrix
        self.board = deepcopy(board)

        #Append the move history to an array
        self.history = [] + history + [self.board] 
    
        
    def children(self):        # returns the possible moves

        moves = [self.try_move_down] # list of move functions

        children = []
        for row in range(len(self.board)):          # will try to move every piece in every direction and return the possible moves
            for col in range(len(self.board[row])):
                for func in moves:
                    child = func(row, col)
                    if child:
                        children.append(child)
        return children
    
    def is_objective(self):                 # checks if the puzzle is complete
        if self.board[3][1]==4 and self.board[4][2]==4:
            return True
        return False

In the moves list there are 4 different directions, but they are structured very similarly. I will include only 1 of the relevant moves methods:

def try_move_down(self, row, col):              # will check the current piece to see if it can be moved downwards
        if row<4:
            if self.board[row][col]==1:               # if the piece type is 1x1
                if self.board[row+1][col]==0:
                    return self.move_down(row, col)
                else:
                    return None
            elif self.board[row][col]==4:             # if the piece type is 2x2
                if (col>0 and self.board[row][col-1]==4) or (row>0 and self.board[row-1][col]==4):
                    return None
                if self.board[row+2][col]==0 and self.board[row+2][col+1]==0:
                    return self.move_down(row, col)
                else:
                    return None
                        
                        
    def move_down(self, row, col):      # will move the current piece upwards
        state=Klotski(self.board, self.move_history)
        if state.board[row][col]==1:                         # piece type 1x1
            state.board[row][col]=0
            state.board[row+1][col]=1
            
        elif state.board[row][col]==4:                       # piece type 2x2
            state.board[row+2][col]=4
            state.board[row+2][col+1]=4
            state.board[row][col]=0
            state.board[row][col+1]=0
        return state.board

The problem arose when I created this function:

def bfs(problem:Klotski):                                           # problem(Klotski) - the initial state
    queue = [problem]
    visited = set()                                         # to not visit the same state twice

    while queue:
        current = queue.pop(0)
        visited.add(str(current))

        if current.is_objective():                          # found the best solution
            return current

        child_list=current.children()
        for child in child_list:               
            if str(child) not in visited:                        # avoid visiting the same state twice
                queue.append(child)

    return None

When I run this cycle, which should print the number of steps it took to solve the puzzle, as well as the board state at every step:

def print_sequence(state):
    print("Steps:", len(state.history) - 1) # prints the sequence of states
    for move in state.history:
        print(move)
        print()

print_sequence(bfs(Klotski([[1,4,4,1], [1,4,4,1], [1,1,1,1], [1,1,1,1], [1,0,0,1]])))

I get on my terminal this error message:

Traceback (most recent call last):
  File "c:\Users\...\Code.py", line 229, in <module>
    print_sequence(bfs(Klotski([[1,4,4,1], [1,4,4,1], [1,1,1,1], [1,1,1,1], [1,0,0,1]])))
  File "c:\Users\...\Code.py", line 216, in bfs
    if current.is_objective():                          # found the best solution
AttributeError: 'list' object has no attribute 'is_objective'

What should I do to fix my problem?...

P.S.:Sorry for the long post and I'm a beginner. I tried to make the post more readable and shorter in size (even though it's still basically a massive code dump...). Also if there's a shorter way to recreate the problem feel free to tell me.