Create an event Handler in One form and call that event in second form in a winforms Project

I have a winform project.

Created an event like:

public delegate voidUserJoin();
public event UserJoin Newjoin;
On the method:
public override void HandleAction()
{
NewJoin();
}
Second form:
private void MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
		{  MyController.Newjoin += new FormController.UserJoin(UserEnter);
		}
public void UserEnter()         {
//some code here
}
When i run the solution getting the Null Reference exception.
Any help?