记事本查找功能问题

做记事本，卡到查找了，求教！！！
查找功能相关代码及记事本附件，求大神指教，怎么把我的查找窗体关掉还不影响使用
主窗体：
    public partial class FrmNote : Form
    {
        private FrmFind ff;
        public FrmNote()
        {
            InitializeComponent();
            ff = new FrmFind();
            ff.Show(this);
            //ff.Hide();     --查找窗体不能关闭，一旦关闭，再查找的时候就出现“未将对象引用设置到对象的实例”异常
        }
        private void 查找ToolStripMenuItem_Click(object sender, EventArgs e)
        {
            ff = new FrmFind();
            ff.Show();
        }
         //查找方法
        public int currentPos=-1;//初始化光标当前位置
        int findPos;//光标开始查找的位置
        public void Search(string s)
        {
            
            //获取光标开始查找的位置
            if (currentPos == -1)
            {
                findPos = tbContent.SelectionStart;
            }
            else
            {
                //获得当前光标位置，并设置新的光标位置，用于查找下一处
                findPos = currentPos + s.Length;
            }
            //判断是否查找到
            currentPos = tbContent.Text.IndexOf(s,findPos);//从findPos处开始查找文本s，查找到则返回索引位置value，否则返回-1,value的索引是选中文本开头
            
            if (currentPos == -1)
            {
                MessageBox.Show("找不到 “" + s+"”", "记事本", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else
            {
                //选中查找文本
                tbContent.Select(currentPos, s.Length);
            }

        }
    }
查找窗体：
   public partial class FrmFind : Form
    {
        private FrmNote fn;
        public FrmFind()//构造函数
        {
            InitializeComponent();

        }
        private void FrmFind_Load(object sender, EventArgs e)
        {
            //使标题栏图标不显示
            this.ShowIcon = false;
            this.Activate();

            this.btnNext.Enabled = false;
        }

        public string str = "";
        private void btnNext_Click(object sender, EventArgs e)
        {
            //获得FrmNote窗体的引用
            fn = this.Owner as FrmNote;
            str = fn.tbContent.Text;
            fn.Search(tbFind.Text);
            
        }
    }
主窗体设计器：
 public System.Windows.Forms.TextBox tbContent;
记事本