什么错了？以及如何解决？？？我正在努力学习一门新课程。运行时收到错误消息:

 public async Task btn1_ClickAsync(object sender, RoutedEventArgs e)
            {
                txtState.Text = "btn1_Click started";
               await LongRunningFunc1();
                txtState.Text = "btn1_Click finished";

            }

        private async Task LongRunningFunc1()
        {
            txt1.Text = "Processing 1 .....";
            btn1.Content = "Wait";
            await Task.Delay(5000);
            txt1.Text = "Hello From Func1";
            btn1.Content = "Click";
        }

wpf设计器:

<Grid>
        <TextBlock Name="txtState" Margin="265,10,274,356"/>
        <TextBlock Name="txt1" Height="50" RenderTransformOrigin="0.966,-0.95" Margin="281,68,320,301"/>
        <TextBlock Name="txt2" Height="50" Margin="253,238,274,131" />
        <Button Name="btn1" Background="Red" Margin="281,127,300,208" Click="btn1_ClickAsync"></Button>
        <Button Name="btn2" Background="Aqua" Margin="298,309,311,27"></Button>
    </Grid>