private void Form_KeyPress(object sender, KeyPressEventArgs e)
{
    if (e.KeyChar == (char)Keys.Enter && this.ActiveControl != theRichTextBox)
    {
        this.DialogResult = DialogResult.OK;
    }
}

private void RichTextBox_Enter(object sender, EventArgs e)
{
    this.AcceptButton = null;
}

private void RichTextBox_Leave(object sender, EventArgs e)
{
    this.AcceptButton = btnAccept;
}