Unity NullReferenceException: Object reference not set to an instance of an object

Question

Every time I click play in Unity, I get this error.

NullReferenceException: Object reference not set to an instance of an object
LobbyManager.PlayerCountUpdate () (at Assets/Scripts/Lobby/LobbyManager.cs:213)
LobbyManager.Update () (at Assets/Scripts/Lobby/LobbyManager.cs:209)

I have tried the following to ensure the ERROR does not pop up:

1. I have ensured the inspector with public variables has been allocated with the assigned GameObject.
2. I tried changing roomSize = PhotonNetwork.CurrentRoom.MaxPlayers; to the start function since its not a variable which require updating but the error still pops up!

Here is My code:

public int playerCount;
int roomSize;


private void Update()
{
    //minimum of 2 players in the room && the player who is clicking the start button is the masterclient
    if (PhotonNetwork.IsMasterClient && PhotonNetwork.CurrentRoom.PlayerCount >= 2)
    {
        playButton.SetActive(true);
    }
    else
    {
        playButton.SetActive(false);
    }

    PlayerCountUpdate();
}
public void PlayerCountUpdate()
{
    roomSize = PhotonNetwork.CurrentRoom.MaxPlayers;
    playerCount = PhotonNetwork.PlayerList.Length;
    roomCountDisplay.text = playerCount + " / " + roomSize;
}