（通用Windows App）System.NullReferenceException：对象引用未设置为对象的实例

当我尝试运行非常简单的通用Windows应用程序时，会弹出上述异常。我尝试了几次以修复它，但它仍然存在。我真的是编程新手，所以如果有人可以帮助我将非常高兴。这是我主页的设计：

<UserControl x:Class="simpleCalc.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:Test_App"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}"
d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="#FFE2D7CC">
    <TextBlock x:Name="textBlock" HorizontalAlignment="Left" Margin="13,50,0,0" Text="Километри" 
TextWrapping="Wrap" VerticalAlignment="Top" Height="31" Width="114" FontFamily="Times New Roman" 
FontSize="22" FontWeight="Bold"
FontStyle="Italic"/>
    <TextBlock x:Name="textBlock1" HorizontalAlignment="Left" Margin="36,103,0,0" Text="Литри" 
TextWrapping="Wrap" VerticalAlignment="Top" Height="32" Width="114" FontFamily="Times New Roman" 
FontSize="22" FontWeight="Bold"
FontStyle="Italic"/>
    <TextBlock x:Name="textBlock2" HorizontalAlignment="Left" Margin="16,147,0,0" Text="Консумация" 
TextWrapping="Wrap" VerticalAlignment="Top" Height="31" Width="114" FontFamily="Times New Roman" 
FontSize="22" FontWeight="Bold"
FontStyle="Italic"/>
    <TextBox x:Name="mileage" HorizontalAlignment="Left" Height="28" Margin="149,47,0,0" 
TextWrapping="Wrap" Text="100" VerticalAlignment="Top" Width="120" Background="#FFD6F6F9"/>
    <TextBox x:Name="liters" HorizontalAlignment="Left" Height="31" Margin="149,97,0,0" 
TextWrapping="Wrap" Text="10" VerticalAlignment="Top" Width="120" Background="#FFD2EFF1"/>
    <TextBox x:Name="cons" HorizontalAlignment="Left" Height="27" Margin="149,145,0,0" 
TextWrapping="Wrap" Text="10" VerticalAlignment="Top" Width="120" Background="#FFC5EAEE"/>
    <TextBlock x:Name="textBlock3" HorizontalAlignment="Left"
Margin="274,147,0,0" TextWrapping="Wrap" Text="1/100 km" VerticalAlignment="Top"/>


    <Button x:Name="button1" Content="ИЗЧИСЛИ" HorizontalAlignment="Left"
Margin="149,199,0,0" VerticalAlignment="Top" Width="120" Click="Button_Click"
Height="35" Foreground="#FF3A2121" Background="#FFF32424">
        <Button.BorderBrush>
            <LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
                <GradientStop Color="#FFA3AEB9" Offset="0"/>
                <GradientStop Color="#FF8399A9" Offset="0.375"/>
                <GradientStop Color="#FF718597" Offset="0.375"/>
                <GradientStop Color="#FFEE3760" Offset="1"/>
            </LinearGradientBrush>
        </Button.BorderBrush>
    </Button>
</Grid>

</UserControl>

这是功能代码：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Runtime.InteropServices.WindowsRuntime;
using Windows.Foundation;
using Windows.Foundation.Collections;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Controls.Primitives;
using Windows.UI.Xaml.Data;
using Windows.UI.Xaml.Input;
using Windows.UI.Xaml.Media;
using Windows.UI.Xaml.Navigation;

namespace simpleCalc
{
    public partial class MainPage : UserControl
    {
       public MainPage()
        {
        InitializeComponent();
        }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        double dMileage = Double.Parse(mileage.Text);
        double dLiters = Double.Parse(liters.Text);
        double dRes = 100 * (dLiters/dMileage);
        cons.Text = dRes.ToString("##.##");
    }
}

}当我运行应用程序（在这种情况下，简单的加油计算）时，它将在此处引发异常：

if (e.PrelaunchActivated == false)
        {
            if (rootFrame.Content == null)
            {
                ***rootFrame.Navigate(typeof(MainPage), e.Arguments);***
            }