- Fix Network Store calling unregister on already unregistered callbacks

[krllus]

On android API 28, the NetworkStore calls unregister on an already unregistered callback, causing the app to crash;

there is no registered issue for this;

To Do

• add “WIP” to the PR title if pushing up but not complete nor ready for review
• [ ] double check the original issue to confirm it is fully satisfied
• [ ] add testing notes and screenshots in PR description to help guide reviewers
• [ ] add unit tests
  • optional: consider adding instrumentation (integration/UI) tests
• consider running this branch in a debug simulator and check for memory leak notifications or any warnings
• [ ] request the "UX" team perform a design review (if/when applicable)
• [ ] make sure CI builds are passing (e.g.: fix lint and other errors)
• [ ] check on the accessibility of any added UI