Don't run hide on tapped code if window is null

[PureWeen]

Description of Change

If navigation isn't animated the navigated event will sometimes fire on the incoming page before IsFocused has transitioned to false on the previous page. This could also happen if there's a bug with Focus propagation so it's important to guard against window being null.

Points to consider around if we should backport.

• the crash will only happen on android if you enable HideSoftInputOnTapped which is new functionality added for NET8
• It only crashes when navigating backwards without animation. By default, shell navigates back without animation if you don't use the parameter to tell it to use animation.
• technically you can work around it by setting HideSoftInputOnTapped false before navigating back
• It's a fairly light change that touches a very isolated area of the code

Issues Fixed

Fixes #17681

[PureWeen]

/backport to release/8.0.1xx-rc2

[github-actions]

Started backporting to release/8.0.1xx-rc2: https://github.com/dotnet/maui/actions/runs/6394127933