NullReferenceException in ShellToolbarAppearanceTracker.SetColors on Android

[BioTurboNick]

Description

Had 3 reports of crashes in this method from two devices, on Android 9 and 10, via AppCenter

Here's the code in question:

maui/src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellToolbarAppearanceTracker.cs

Lines 34 to 39 in a3b135f

	protected virtual void SetColors(AToolbar toolbar, IShellToolbarTracker toolbarTracker, Color foreground, Color background, Color title)
	{
	_shellContext.Shell.Toolbar.BarTextColor = title ?? ShellRenderer.DefaultTitleColor;
	_shellContext.Shell.Toolbar.BarBackground = new SolidColorBrush(background ?? ShellRenderer.DefaultBackgroundColor);
	_shellContext.Shell.Toolbar.IconColor = foreground ?? ShellRenderer.DefaultForegroundColor;
	}

Is the MAUI library using nullable types? If not, there's no verification that _shellContext is not null when the object is constructed. And if it's at all possible to be accessed after being disposed, that would also produce this exception.

Even if _shellContext is not null, is .Shell, or .Shell.Toolbar guaranteed to not be null?

Steps to Reproduce

Unknown

Link to public reproduction project repository

n/a

Version with bug

7.0 (current)

Last version that worked well

Unknown/Other

Affected platforms

Android

Affected platform versions

Android 9-10

Did you find any workaround?

No response

Relevant log output

Full stack trace:

Microsoft.Maui.Controls.Platform.Compatibility.ShellToolbarAppearanceTracker.SetColors(Toolbar toolbar, IShellToolbarTracker toolbarTracker, Color foreground, Color background, Color title)
Microsoft.Maui.Controls.Platform.Compatibility.ShellToolbarAppearanceTracker.SetAppearance(Toolbar toolbar, IShellToolbarTracker toolbarTracker, ShellAppearance appearance)
Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.SetAppearance(ShellAppearance appearance)
Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.Microsoft.Maui.Controls.IAppearanceObserver.OnAppearanceChanged(ShellAppearance appearance)
Microsoft.Maui.Controls.Shell.Microsoft.Maui.Controls.IShellController.AppearanceChanged(Element source, Boolean appearanceSet)
Microsoft.Maui.Controls.ShellSection.SendAppearanceChanged()
Microsoft.Maui.Controls.ShellSection.UpdateDisplayedPage()
Microsoft.Maui.Controls.ShellSection.OnVisibleChildAdded(Element child)
Microsoft.Maui.Controls.ShellSection.OnChildAdded(Element child)
Microsoft.Maui.Controls.BaseShellItem.AddLogicalChild(Element element)
Microsoft.Maui.Controls.ShellSection.AddPage(Page page)
Microsoft.Maui.Controls.ShellSection.OnPushAsync(Page page, Boolean animated)
Microsoft.Maui.Controls.ShellSection.PushStackOfPages(List`1 pages, Nullable`1 animate)
Microsoft.Maui.Controls.ShellSection.GoToAsync(ShellNavigationRequest request, ShellRouteParameters queryData, IServiceProvider services, Nullable`1 animate, Boolean isRelativePopping)
Microsoft.Maui.Controls.ShellNavigationManager.GoToAsync(ShellNavigationParameters shellNavigationParameters, ShellNavigationRequest navigationRequest)

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[PureWeen]

@BioTurboNick those should never be null so this probably indicates that something isn't unwiring correctly further up the chain. Can you provide some added context about the structure of your app?

1. do you ever swap out the main page or is it always just shell?
2. if the user backgrounds the app and then foregrounds it does shell get recreated?
3. what does your appshell xaml look like?
4. Do you have any ideas what that GotoAsync call is doing? Is it just pushing pages? is it switching to a different shell element?

[BioTurboNick]

1. Always just Shell
2. It shouldn't, haven't touched the default creation/suspend/restore logic
3. See below
4. I'm not sure why the name of the function I'm actually calling is dropped, but this where it must be invoked:

await AppShell.Current.Navigation.PushAsync(new HemocytometerPage(pageViewModel));

This line is present in the Execute method of the command invoked when one of the MenuItem entries is clicked.

<?xml version="1.0" encoding="UTF-8" ?>
<Shell
    x:Name="root"
    x:Class="HemocytometerSidekick.AppShell"
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:local="clr-namespace:HemocytometerSidekick"
    xmlns:views="clr-namespace:HemocytometerSidekick.Views"
    xmlns:strings="clr-namespace:HemocytometerSidekick.Resources.Strings"
    FlyoutBackgroundColor="{AppThemeBinding Dark={StaticResource DarkBackgroundColor}}">

    <Shell.FlyoutHeader>
        <Grid
            BackgroundColor="{AppThemeBinding Light={StaticResource Primary}, Dark={StaticResource PrimaryMediumDark}}"
            ColumnDefinitions="10,80,*,10"
            RowSpacing="5"
            RowDefinitions="30,80,Auto"
            Padding="0, 0, 0, 10">

            <Image
                Grid.Row="1"
                Grid.Column="1"
                Source="menulogo.png" />

            <Label
                Grid.Column="1"
                Grid.Row="2"
                Grid.ColumnSpan="2"
                TextColor="White"
                FontSize="20"
                Text="{x:Static strings:AppResources.AppName}" />
            
        </Grid>
    </Shell.FlyoutHeader>

    <ShellContent
        Title="{x:Static strings:AppResources.Home}"
        ContentTemplate="{DataTemplate views:IntroPage}"
        Route="IntroPage">
        <ShellContent.Icon>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE80F;" />
        </ShellContent.Icon>
    </ShellContent>

    <MenuItem
        Text="{x:Static strings:AppResources.Hemocytometer}"
        BindingContext="{x:Reference root}"
        Clicked="Hemocytometer_Clicked">
        <MenuItem.IconImageSource>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE8EF;" />
        </MenuItem.IconImageSource>
    </MenuItem>

    <MenuItem
        Text="{x:Static strings:AppResources.BloodDifferential}"
        BindingContext="{x:Reference root}"
        Clicked="BloodDifferential_Clicked">
        <MenuItem.IconImageSource>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE8EF;" />
        </MenuItem.IconImageSource>
    </MenuItem>
    
    <ShellContent
        Title="{x:Static strings:AppResources.Results}"
        ContentTemplate="{DataTemplate views:ResultsPage}">
        <ShellContent.Icon>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE8BC;" />
        </ShellContent.Icon>
    </ShellContent>

    <MenuItem
        Text="{x:Static strings:AppResources.Feedback}"
        BindingContext="{x:Reference root}"
        Command="{Binding SendFeedbackCommand}">
        <MenuItem.IconImageSource>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE939;" />
        </MenuItem.IconImageSource>
    </MenuItem>

    <ShellContent
        Title="{x:Static strings:AppResources.Settings}"
        BindingContext="{x:Reference root}"
        ContentTemplate="{DataTemplate views:SettingsPage}">
        <ShellContent.Icon>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE713;" />
        </ShellContent.Icon>
    </ShellContent>

    <MenuItem
        Text="{x:Static strings:AppResources.MoreInformation}"
        BindingContext="{x:Reference root}"
        Command="{Binding MoreInformationCommand}">
        <MenuItem.IconImageSource>
            <FontImageSource
                FontFamily="SegoeFluentIcons"
                Color="{AppThemeBinding Light={StaticResource Gray900}, Dark={StaticResource DarkThemeLightTextColor}}"
                Glyph="&#xE946;" />
        </MenuItem.IconImageSource>
    </MenuItem>

</Shell>

[BioTurboNick]

Some other crashes recorded by AppCenter definitely point to something wrong with disposal:

Microsoft.Maui.Controls.Platform.Compatibility.ShellToolbarTracker.Dispose(Boolean disposing)
Java.Lang.Object.Java.Interop.IJavaPeerable.Disposed()
Java.Interop.JniRuntime.JniValueManager.DisposePeer(IJavaPeerable )
Java.Lang.Object.Dispose()
Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.Destroy()
Microsoft.Maui.Controls.Platform.Compatibility.ShellSectionRenderer.OnDestroy()
AndroidX.Fragment.App.Fragment.n_OnDestroy(IntPtr , IntPtr )
Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V(_JniMarshal_PP_V , IntPtr , IntPtr )

[JoacimWall]

Found a workaround that works for me.
App.xaml.cs
protected async override void OnResume()
{
if (DeviceInfo.Platform == DevicePlatform.Android)
{
Shell.SetBackgroundColor(this, AppColors.Primary500Color);
Shell.SetForegroundColor(this, AppColors.WhiteColor);
Shell.SetTabBarBackgroundColor(this, AppColors.Primary800Color);
Shell.SetTabBarForegroundColor(this, AppColors.PrimaryAccentColor);
Shell.SetTabBarUnselectedColor(this, AppColors.WhiteColor);
Shell.SetTabBarTitleColor(this, AppColors.PrimaryAccentColor);
}
}