Object reference not set to an instance of an object in a label? c# [duplicate]

This question already has an answer here:

• What is a NullReferenceException and how do I fix it? 17 answers

I am trying to pass a username into a label in a windows form application from a login however it says Object reference not set to an instance of an object

in lblWelcome it says null but in the string msg it says the right logged in username why is this happening?

login:

i initialize it in the public part of the login

string msg;

then this is the if ssuccessul login

 int i = ds.Tables[0].Rows.Count;
                if (i == 1)
                {
                    msg = "Welcome " + txtUsername.Text;
                    this.Hide();
                    Welcome f2 = new Welcome(msg);
                    f2.Show();
                    ds.Clear();

                }

formwelcome

 public Welcome()
    {
        InitializeComponent();
    }

    public Welcome(string msg)
    {
        InitializeComponent();
        lblWelcome.Text = msg;
    }