MissingReferenceException on second time in gameplay scene

bur_ff

I have a game that has a lobby screen which has only one button "Play". When we clicked it, it will join a room randomly like this:

public void OnPlayClick()
{
PhotonNetwork.JoinRandomRoom()
}

When everything is ready, I call a function to load the game scene:

public void LoadGameScene()
{
PhotonNetwork.CurrentRoom.IsOpen = false;
PhotonNetwork.CurrentRoom.IsVisible = false;

PhotonNetwork.LoadLevel("GameScene");
}

The game scene is loading and everything is ok in the first time. The problem occurs after the player clicks to "LeaveRoom" button. I made leave room and return to lobby procedure like that:

public void OnLeaveRoomClick()
{
PhotonNetwork.DestroyPlayerObjects(PhotonNetwork.LocalPlayer);
PhotonNetwork.LeaveRoom();
}

public override void OnLeftRoom()
{
PhotonNetwork.Disconnect();
}

public override void OnDisconnected(DisconnectCause cause)
{
SceneManager.LoadScene("LobbyScene");
}

When we are in the lobby scene, if we click the play button again, everything I said above is working and the game scene opens successfully but a bunch of error pops up said: "MissingReferenceException: The object of type '...' has been destroyed but you are still trying to access it."

I think something was not successfully destroyed when leaving room from the first time. But I don't know exactly. How could this happen?

Every help appreciated. Thanks.