Fix - Selected Hyperion instance isn't running (#1357)

[Lord-Grey]

Summary

• Fixed "Selected Hyperion instance isn't running" issue which happens, if backend instance definitions do not fit to the UI
  (caused by new/deleted database). Now there is automatic fall-back to the default instance rather than an error presented.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of web configuration, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing setups:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's body (e.g. Fixes: #xxx[,#xxx], where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated (docs/docs/en)
• Related tests have been updated

PLEASE DON'T FORGET TO ADD YOUR CHANGES TO CHANGELOG.MD

• Yes, CHANGELOG.md is also updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:
Fixes #1357

[hyperion-project]

Hello @Lord-Grey 👋

I'm the Hyperion Project Bot and I want to thank you for
contributing to Hyperion with your pull requests!

To help you and other users test your pull requests faster,
I'll create a link for you to your workflow artifacts.

🔗 https://github.com/hyperion-project/hyperion.ng/actions/runs/1354389358

Of course, if you make changes to your PR, I will create a new link.

Best regards,
Hyperion Project