Why did my XAML suddenly crash?

I added a "Basic Page" to my project in a folder ("View") that I created within the project.

At first all was well, but "all of a sudden" the design view failed and displayed:

System.ObjectDisposedException Safe handle has been closed

This is the XAML (I hadn't changed a thing in the default XAML generated yet):

<common:LayoutAwarePage
    x:Name="pageRoot"
    x:Class="TimeAndSpaceLines.View.SectionPage"
    DataContext="{Binding DefaultViewModel, RelativeSource={RelativeSource Self}}"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TimeAndSpaceLines.View"
    xmlns:common="using:TimeAndSpaceLines.Common"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Page.Resources>

        <!-- TODO: Delete this line if the key AppName is declared in App.xaml -->
        <x:String x:Key="AppName">My Application</x:String>
    </Page.Resources>

    <!--
        This grid acts as a root panel for the page that defines two rows:
        * Row 0 contains the back button and page title
        * Row 1 contains the rest of the page layout
    -->
    <Grid Style="{StaticResource LayoutRootStyle}">
        <Grid.RowDefinitions>
            <RowDefinition Height="140"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>

        <!-- Back button and page title -->
        <Grid>
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="Auto"/>
                <ColumnDefinition Width="*"/>
            </Grid.ColumnDefinitions>
            <Button x:Name="backButton" Click="GoBack" IsEnabled="{Binding Frame.CanGoBack, ElementName=pageRoot}" Style="{StaticResource BackButtonStyle}"/>
            <TextBlock x:Name="pageTitle" Grid.Column="1" Text="{StaticResource AppName}" Style="{StaticResource PageHeaderTextStyle}"/>
        </Grid>

        <VisualStateManager.VisualStateGroups>

            <!-- Visual states reflect the application's view state -->
            <VisualStateGroup x:Name="ApplicationViewStates">
                <VisualState x:Name="FullScreenLandscape"/>
                <VisualState x:Name="Filled"/>

                <!-- The entire page respects the narrower 100-pixel margin convention for portrait -->
                <VisualState x:Name="FullScreenPortrait">
                    <Storyboard>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="backButton" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource PortraitBackButtonStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
                    </Storyboard>
                </VisualState>

                <!-- The back button and title have different styles when snapped -->
                <VisualState x:Name="Snapped">
                    <Storyboard>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="backButton" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource SnappedBackButtonStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
                        <ObjectAnimationUsingKeyFrames Storyboard.TargetName="pageTitle" Storyboard.TargetProperty="Style">
                            <DiscreteObjectKeyFrame KeyTime="0" Value="{StaticResource SnappedPageHeaderTextStyle}"/>
                        </ObjectAnimationUsingKeyFrames>
                    </Storyboard>
                </VisualState>
            </VisualStateGroup>
        </VisualStateManager.VisualStateGroups>
    </Grid>
</common:LayoutAwarePage>

Note: I did try removing the "appname" line according to the "TODO" (it is in app.xaml) but that made no difference. The app builds successfully via F6, but attempts to reload the designer fail.

A second attempt, this time using a Blank xaml page, and then pasting in some XAML I had cobbled together in Kaxaml, also fails, this time with the err msg: "System.Exception Package could not be registered. (Exception from HRESULT: 0x80073CF6)"

This XAML is (truncated):

<Page
    x:Class="TimeAndSpaceLines.View.SectionN"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TimeAndSpaceLines.View"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <Grid MinHeight="600" ShowGridLines="True" Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="320">
                    </ColumnDefinition>
                    <ColumnDefinition MaxWidth="800">
                    </ColumnDefinition>
                </Grid.ColumnDefinitions>
                <Grid.RowDefinitions>
                    <RowDefinition>
                    </RowDefinition>
    . . .

???

I had same issue, seems like problem is that VS trying to "deploy" any your application before it's starting debugging. But this deployment can be forbidden by Windows Local Group Policies. So you need to do next steps:

1. Press Win + X to open Run dialog, type gpedit.msc and press Enter;
2. Go to Computer Configuration > Administrative Templates > Windows Components > App Package Deployment;
3. Double-click ‘Allow all trusted apps to install’, select Enabled and choose OK;

It helped for me.