Handles exception thrown when the theme has not been initialised yet

[antonis]

Fixes #15300

Based on #15308 and targeting release/18.2

Description

This PR fixes a NullPointerException introduced in #15134

To test:
I was not able to reproduce the conditions that lead to a crash. Thus I only validated the fix by triggering a null value for the editorTheme at debug time.

A few edge cases I tested include:

• Starting the editor quickly after a clean install of the app
• Starting the editor quickly in offline mode after a clean install of the app

Regression Notes

1. Potential unintended areas of impact
   N/A

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

3. What automated tests I added (or what prevented me from doing so)
   The complexity of EditPostActivity makes it hard to add an automated tests for this part of the code.

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.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-15311-build-115244.apk
• WordPress-pr-15311-build-115244.apk

[peril-wordpress-mobile]

	Warnings
⚠️	This PR is assigned to a milestone which is closing in less than 4 days Please, make sure to get it merged by then or assign it to a later expiring milestone

Generated by 🚫 dangerJS

[develric]

Hey @antonis 👋 , this LGTM! Even if not consistently, I was able to get the null from getEditorThemeForSite when using a pure self-hosted site on which I've opted out from Block Editor and especially when creating a post just after app starting up. Cannot be sure that is the only edge case, but regardless the fix worked for me 👍 .

I'll let you align with @AliSoftware to make this PR through the beta update, but let me know if any action needed from my side 🙇 .

[mkevins]

Hey @antonis , this LGTM! Even if not consistently, I was able to get the null from getEditorThemeForSite when using a pure self-hosted site on which I've opted out from Block Editor and especially when creating a post just after app starting up. Cannot be sure that is the only edge case, but regardless the fix worked for me .

I'll let you align with @AliSoftware to make this PR through the beta update, but let me know if any action needed from my side .

Thanks for testing this and finding steps to reproduce the original issue @develric . I had tried a few things to find steps, but nothing was reproducing it, I hadn't thought to opt out of the block editor on a self-hosted site.

Code looks good, thanks for fixing Antonis! 👍

[antonis]

@develric, @mkevins thank you both for reviewing this so quickly 🙇
I'll proceed with merging this in release/18.2

[AliSoftware]

This fix is now part of the new 18.2-rc-3 beta, thanks all 🎉