在c#中导航表单时抛出ObjectDisposeException

我想在表单中导航所以当我点击buttonclient时，我进入Form2然后当我点击control 'x'时，我回到Form1，我得到:

(fk为Form2)

   private void button1_Click(object sender, EventArgs e) {
                if (fk == null)
                    fk = new OknoKlient();
                fk.Tag = this;
                fk.Show(this);//here is ObjectDisposedException
                Hide();
            }

然后在Form2

 protected override void OnFormClosing(FormClosingEventArgs e) {
            if (e.CloseReason == CloseReason.WindowsShutDown) return;
            var form1 = (Form1)Tag;
            form1.Show();
            Hide();
            // DO WHATEVER HERE
        }

当我单击button1时，Form2 fk打开，然后我通过x控件关闭它，然后再次单击button1，我得到例外ObjectDisposedException。

protected override void OnFormClosing(FormClosingEventArgs e) {
    if (e.CloseReason == CloseReason.WindowsShutDown) return;
    e.Cancel = true; // <---------------------- this
    var form1 = (Form1)Tag;
    form1.Show();
    Hide();
    // DO WHATEVER HERE
}