Fix issue with Geodatabase transactions sample when navigating away

[duffh]

Description

A missing null check resulted in a thrown exception when navigating away from the Geodatabase transactions sample.

Type of change

• Bug fix

Platforms tested on

• WPF .NET 6
• WPF Framework
• WinUI

Checklist

• Self-review of changes
• All changes work as expected on all affected platforms
• There are no warnings related to changes
• Code is commented and follows .NET conventions and standards
• Codemaid and XAML styler extensions have been run on every changed file
• No unrelated changes have been made to any other code or project files

[williambohrmann3]

Thanks! tested both sample implementations.

[williambohrmann3]

Quite rare, have only been able to repro this once after about 10 attempts:
getting these errors after changing samples then returning to GeodatabaseTransactions:

Might be worth looking into but I don't think this is related to your PR. Very odd I only could repro this once.