A targeted TeachingTip does not get unloaded when its target does

[SavoySchuler]

Describe the bug

A targeted TeachingTip does not get unloaded when its target gets unloaded.

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Add a targeted teaching tip to a page-specific instance of a UI element.
2. Navigate to that page and prompt the tip to open.
3. Navigate away from that page.

Expected behavior

I expect the teaching tip to get unloaded when its target gets unloaded.

Actual behavior

The target gets unloaded but the teaching tip remains on-screen.

Screenshots

Version Info

NuGet package version:
Microsoft.UI.Xaml 2.1.190319001-prerelease

Windows 10 version	Saw the problem?
Insider Build (18865)	Yes
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)
Anniversary Update (14393)

Device form factor	Saw the problem?
Desktop	Yes
Mobile
Xbox
Surface Hub
IoT

Additional context

[jevansaks]

Agreed that when the TeachingTip itself gets unloaded we should hide the popup. I'm not sure about watching the target.

[marcelwgn]

I will fix this :)