DisplayActionSheet later

[cshung]

Description of Change

There are a couple of issues with this bug.

1. We need to wait further before handling the pending actions, and
2. Async operation needs to be completed no matter what.

The description reflects the execution sequence:

By the time DisplayActionSheet is called in OnAppearing, the Page knows it IsPlatformEnabled is false, so it adds this task into a _pendingActions list.

This list will be drained when IsPlatformEnabled is set to true during CreatePlatformWindow. While the Handler is available, the underlying WinUI objects are not on the VisualTree yet, so it isn't really ready to run the _pendingActions.

In particular, at that point of time, the page is not null, but its parent is null, so the call will silently terminate without signaling the async result, and the async call would appear to be forever executing. So I changed the code to SetResult(null) even in those cases, those case should not happen, but it could if the user calls the API inappropriately.

If we give the page instead of its parent to actionSheet.ShowAt(), the call would fail with an E_INVALIDARG returned from the underlying native code, presumably because it is not in the VisualTree as the comment indicates. The exception handler would try to use UI.Xaml.Window.Current.Content instead, but UI.Xaml.Window.Current is null at that point of time, further confirming it is too early to make this call.

I experimented with moving the flushing of the _pendingActions to some time later, such as OnSizeAllocated, this will work. This is not ideal though, as the method is called quite frequently on resizing. I am wondering if there is a better place to flush them. It has to be at a point where the objects are already in the VisualTree.

The code is tested on both Windows and Android emulator, at the moment I am not able to test on other platforms. I assumed that OnLoaded should function properly on all platforms.

Issues Fixed

Fixes #3726