[Reader] Fix UninitializedPropertyAccessException in ReaderFragment

[RenanLukas]

Fixes #20142

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To Test:

I couldn't reproduce this crash. The ViewModel is initialized when ReaderFragment#onViewCreated is called. The crash log tells us that ReaderFragment#onSaveInstanceState was called before ReaderFragment#onViewCreated, since ViewModel was not initialized.

There wasn't a single time where I was able to make onSaveInstanceState be called before onViewCreated:

com.jetpack.android.prealpha         E  RL-> ReaderFragment initViewModel
com.jetpack.android.prealpha         E  RL-> WPMainActivity onSaveInstanceState
com.jetpack.android.prealpha         E  RL-> ReaderFragment onSaveInstanceState

onSaveInstanceState is called after onStop in recent versions and might be called before or after (no guarantees on order) onPause in older versions. This means for this crash to happen, the Activity should've been stopped before onViewCreated is called.

For now I've added a check to see if the ViewModel property was initialized before using it to avoid the crash.

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Regression Notes

1. Potential unintended areas of impact

   • None

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • Manual testing

3. What automated tests I added (or what prevented me from doing so)

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PR Submission Checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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Testing Checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• Talkback.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• Large and small screen sizes. (Tablet and smaller phones)
• Multi-tasking: Split screen and Pop-up view. (Android 10 or higher)

[dangermattic]

	1 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20143-6b94752
	Commit	6b94752
	Direct Download	wordpress-prototype-build-pr20143-6b94752.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr20143-6b94752
	Commit	6b94752
	Direct Download	jetpack-prototype-build-pr20143-6b94752.apk

Note: Google Login is not supported on these builds.

[daniloercoli]