[BUG] FileSaver on iOS crashing when started from a modal

[softlion]

Is there an existing issue for this?

• I have searched the existing issues

Did you read the "Reporting a bug" section on Contributing file?

• I have read the "Reporting a bug" section on Contributing file: https://github.com/CommunityToolkit/Maui/blob/main/CONTRIBUTING.md#reporting-a-bug

Current Behavior

Crash log:

Attempt to present <UIDocumentPickerViewController: 0x1252d06a0> on 
<Microsoft_Maui_Controls_Platform_Compatibility_ShellFlyoutRenderer: 0x10a9aa160> (from 
<Microsoft_Maui_Controls_Platform_Compatibility_ShellFlyoutRenderer: 0x10a9aa160>) which is already presenting 
<Microsoft_Maui_Controls_Platform_ModalWrapper: 0x139806360>.

Expected Behavior

Don't crash.

Or at least give a way to choose the UIViewController on which to display the documentPickerViewController.

Steps To Reproduce

<ContentPage ...
             Shell.NavBarIsVisible="False"
             Shell.PresentationMode="ModalAnimated"
             xmlns:iOsSpecific="clr-namespace:Microsoft.Maui.Controls.PlatformConfiguration.iOSSpecific;assembly=Microsoft.Maui.Controls"
             iOsSpecific:Page.UseSafeArea="True"
             iOsSpecific:Page.ModalPresentationStyle="Automatic"
             >
    <VerticalStackLayout>
          <Button Command="{Binding PickCommand}" />
    </VerticalStackLayout>
</ContentPage>

Code in PickCommand:

            await using var stream = File.OpenRead(finalExportFilePath);
            var fileSaverResult = await FileSaver.SaveAsync(exportTitle, stream, CancellationToken.None);

Link to public reproduction project repository

Needs reproduction

Environment

- .NET MAUI CommunityToolkit: 5.0.0
- OS: iOS
- .NET MAUI: 7.0.81 / 7.0.100 / SDK 7.0.200

Anything else?

The source code is using this key window to display the controller:

		var currentViewController = Microsoft.Maui.Platform.UIApplicationExtensions.GetKeyWindow(UIApplication.SharedApplication)?.RootViewController;
		currentViewController?.PresentViewController(documentPickerViewController, true, null);

Example corner cases that changes the key window hierarchy.

• screen recording is ON.
• screen sharing is ON. For ex using the microsoft store app "AirPlay Screen Mirroring Receiver"
• a "popup" is already presented (this case)

Instead, you should use:

Platform.GetCurrentUIViewController()

 var presentingVc = Platform.GetCurrentUIViewController();
if (presentingVc != null)
{
    UIApplication.SharedApplication.InvokeOnMainThread(() =>
    {
        var yourVC = ...
        presentingVc.PresentViewController(yourVC, true, null);
    });
}
else
    Log?.LogWarning("Input: no window/nav controller on which to display");

[softlion]

Also there is another bug in the implementation.

If currentViewController is null, PresentViewController is not called, and the taskCompetedSource is never completed. Thus freezing the app.

To fix it, you could add:

if (currentViewController == null)
	throw new ("No UIViewController from which to display the document picker");