Buggy python submission (verdict: Time Limit Exceeded):
def clear_maze(sx, sy, gx, gy, maze):

    INF = float("inf")

    field_x_length = len(maze)
    field_y_length = len(maze[0])
    distance = [[INF for i in range(field_y_length)] for j in range(field_x_length)]
    #    distance = [[None]*field_x_length]*field_y_length

    def bfs():
        queue = []

        queue.insert(0, (sx, sy))

        distance[sx][sy] = 0

        while len(queue):
            x, y = queue.pop()

            if x == gx and y == gy:
                break

            for i in range(0, 4):
                nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]

                if 0 <= nx and nx < field_x_length and 0 <= ny and ny < field_y_length:
                    try:
                        if maze[nx][ny] != '#' and distance[nx][ny] == INF:
                            queue.insert(0, (nx, ny))
                            distance[nx][ny] = distance[x][y] + 1
                    except Exception as e:
                        print(nx, ny)
                        print(maze)
                        print(maze[nx][ny])
                        print(distance)
                        exit()

        return distance[gx][gy]

    return bfs()


H, W = map(int, input().split())
maze = []
for w in range(H):
    maze.append([x for x in input()])
ans = 0
for h in range(H):
    for w in range(W):
        if maze[h][w] == ".":
            sx, sy = h, w
            for g in range(H):
                for l in range(w, W):
                    if maze[g][l] == ".":
                        gx, gy = g, l
                        ans = max(clear_maze(sx,sy,gx,gy, maze),ans)
print(ans)


Fixed submission:
def clear_maze(sx, sy, maze):

    INF = float("inf")

    field_x_length = len(maze)
    field_y_length = len(maze[0])
    distance = [[INF for i in range(field_y_length)] for j in range(field_x_length)]
    #    distance = [[None]*field_x_length]*field_y_length

    def bfs():
        queue = []

        queue.insert(0, (sx, sy))

        distance[sx][sy] = 0

        while len(queue):
            x, y = queue.pop()


            for i in range(0, 4):
                nx, ny = x + [1, 0, -1, 0][i], y + [0, 1, 0, -1][i]

                if 0 <= nx and nx < field_x_length and 0 <= ny and ny < field_y_length:
                    try:
                        if maze[nx][ny] != '#' and distance[nx][ny] == INF:
                            queue.insert(0, (nx, ny))
                            distance[nx][ny] = distance[x][y] + 1
                    except Exception as e:
                        print(nx, ny)
                        print(maze)
                        print(maze[nx][ny])
                        print(distance)
                        exit()
        res = 0
        for x in range(field_x_length):
            for y in range(field_y_length):
                if distance[x][y] < float("inf"):
                    res = max(res, distance[x][y])
        return res

    return bfs()


H, W = map(int, input().split())
maze = []
for w in range(H):
    maze.append([x for x in input()])
ans = 0
for h in range(H):
    for w in range(W):
        if maze[h][w] == ".":
            sx, sy = h, w
            ans = max(clear_maze(sx,sy, maze),ans)
print(ans)