I've spent a ridiculous amount of time trying to find an answer to what seems like it should be so easy with no success...

How do I reference Form1 in the below, without using "((Form1)Form1.ActiveForm)"?

That works, but obviously crashes the app if you click off it while something that's using it is working.

Code:
namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            changetext cg1 = new changetext();
            cg1.textbox1();
        }

        public void change_textbox_text(string input)
        {
            this.textBox1.Text = (input);
        }
    }

    class changetext
    {
        public void textbox1()
        {
            ((Form1)Form1.ActiveForm).change_textbox_text("OK");
        }
    }
}