required for the non-static field, method for UserControl2.ButtonClicked

Hi Why i am getting object reference is required for the non-static field for UserControl2.ButtonClicked?

Here's my code below.

Form1

UserControl2.ButtonClicked += new EventHandler(userControl2_ButtonClicked);

    protected void userControl2_ButtonClicked(object sender, EventArgs e)
    {
        flag *= -1;
        if (flag == 1)
            bunifuGradientPanel1.Hide();
        else
            bunifuGradientPanel1.Show();

    }

UserControl:

public event EventHandler ButtonClicked;

private void button1_Click(object sender, EventArgs e)
{
    if (this.ButtonClicked != null)
        this.ButtonClicked(this, e);   
}