C#, System.Reflection.TargetInvocationException with async Task

Question

I got problem during using async Task in C#. When I try to close Form2 during execution Task "DoSomeThing", the Visiual Studio show the next mistake "System.Reflection.TargetInvocationException".How can it be fixed? Form 1 (has button to go to Form2)

public Form1()
        {
            InitializeComponent();
        }

        private void AddWindow_FormClosed(object sender, FormClosedEventArgs e)
        {
            this.Show();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Form2 ukl = new Form2 ();
            ukl.FormClosed += new FormClosedEventHandler(AddWindow_FormClosed);
            ukl.Show();
            this.Hide();
        }

Form2

static CancellationTokenSource cts;
        public Form2()
        {
            InitializeComponent();
        }

        private async void Form2_Load(object sender, EventArgs e)
        {
            cts = new CancellationTokenSource();
            await DoSomeThing(cts.Token);
        }

        async Task DoSomeThing(CancellationToken token)
        {

            while(double.Parse(label1.Text )< 100)
            {
                if (token.IsCancellationRequested)
                {
                    return;
                }
                label1.Text = (double.Parse(label1.Text) + 1).ToString();
                await Task.Delay(100);
            }
        }

        private async void Form2_FormClosed(object sender, FormClosedEventArgs e)
        {
            cts = new CancellationTokenSource();
            cts.Cancel();
            await DoSomeThing(cts.Token);
        }

I tried to use Cancellation Token, but it did't help