RichEditBox file upload crashes Desktop app

[j0shuams]

Describe the bug
When you try to upload a file in the RichEditBox page, the app crashes.
I believe this is an issue with not initializing the file picker* with a window handle.
*The file picker object in RichEditBoxPage.xaml.cs.
See Additional context for related links

Steps to reproduce the bug

Steps to reproduce the behavior:

1. Use the winui3preview branch
2. Open XamlControlsGallery.Desktop.sln in VS
3. Build and run the app
4. Navigate to Text > RichEditBox > 'A custom editor with RichEditBox'
5. Click the file upload button

Expected behavior
A new window should open that allows the user navigation to a RTF file to open.

Actual behavior

The app crashes, shuts down.

Screenshots

Button that causes the crash:

Output window after call to PickSingleFileAsync()

Call stack with breakpoint on PickSingleFileAsync()

Output window upon crash:

Call stack upon crash:

Version Info

CsWinRT Version used: 0.1.0-prerelease.200629.3
WinUI Version used: 3.0.0-preview2.200713.0

Gallery version (found on Settings page):

1.2.27.0

Windows 10 version:

• Insider Build (xxxxx)
• May 2020 Update (19041)
• November 2019 Update (18363)
• May 2019 Update (18362)
• October 2018 Update (17763)
• April 2018 Update (17134)
• Fall Creators Update (16299)
• Creators Update (15063)
• Anniversary Update (14393)

Device form factor:

• Desktop
• Mobile
• Xbox
• Surface Hub
• IoT

Additional context
This is very similar to another issue that was reported

And a potential solution could be derived from this cswinrt example

[stmoy]

@j0shuams - thank you for filing this issue!

CC: @MikeHillberg

[j0shuams]

@MikeHillberg This no longer repros with XCG on latest reunion bits FWIW