fix #1138 File saver and Folder picker

[softlion]

Description of Change

Fix issue #1138 by using the correct method to get a UIController where to display the picker.

Also prevent future potential app freeze by making sure the TaskCompletionSource won't wait on nothing.

Linked Issues

Fixes #1138 , #1145

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• [-] Has tests (if omitted, state reason in description)
• [-] Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• [-] Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pull/XYZ

Additional information

[MartyIX]

(Maybe you would be interested in taking a look at dotnet/maui#13814 and suggesting how to fix it properly there given you understand this topic. 🙏)