fix: Fixed Popup Close call possible NRE.

[HavenDV]

Description of Change

In a situation where the opening and closing of the Popup depends on external factors, and the Close call occurs before the Show call, there is an NRE here since the Handler is null at that moment

Linked Issues

• Fixes #

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

[VladislavAntonyuk]

@HavenDV please create an issue before creating a PR.

[brminnick]

@HavenDV Could you please open an Issue for the bug that you've fixed this PR and include a sample reproduction?

Before we can merge this PR, we need to validate the bug and determine whether this PR fixes its root-cause; we can't do that without a bug report + a reproduction sample.

[HavenDV]

I don't think it's that important. I think it was just a very specific case, so it's easier to close it. Sorry for wasting time