Fixes #1389: Fall back to root VC if no topMostPresented is found

[bjtitus]

Fixes #1389

No topMostPresentedViewController is found when initially launching the app. We should fall back to the root view controller (the MainTabBarController) in these instances.

Before	After

To test

• Build and run on device

• Kill the app

• Add a link (https://pca.st/podcast/eebcc9f0-cec7-013c-5ce5-0acc26574db2) to reminders

• Tap the link

• Verify that the app displays the loading alert and opens to the podcast

• Tap the link again while the app is backgrounded

• Ensure that the podcast opens

Checklist

• I have considered if this change warrants user-facing release notes and have added them to CHANGELOG.md if necessary.
• I have considered adding unit tests for my changes.
• I have updated (or requested that someone edit) the spreadsheet to reflect any new or changed analytics.

[dangermattic]

	1 Warning
⚠️	This PR is assigned to the milestone 7.63 ❄️. The due date for this milestone has already passed. Please assign it to a milestone with a later deadline or check whether the release for this milestone has already been finished.

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