Address NullPointerException crash in AztecHeadingSpan

[SiobhyB]

A potential fix for the crash described at wordpress-mobile/WordPress-Android#18657.

Related PRs

• Gutenberg: [RNMobile] Address NullPointerException crash in AztecHeadingSpan WordPress/gutenberg#56757
• Gutenberg Mobile: [RNMobile] Address NullPointerException crash in AztecHeadingSpan gutenberg-mobile#6435
• Android: Address NullPointerException crash in AztecHeadingSpan WordPress-Android#19724

Description

Sentry has been reporting the following crash for both the WordPress and Jetpack Android apps:

NullPointerException
org.wordpress.aztec.spans.AztecHeadingSpan in chooseHeight

The stack trace for each individual event specifically references one of the following lines (the specific line varies for each event), all of which are within the chooseHeight function and include a reference to previousFontMetrics:

• Line 131
• Line 132
• Line 155
• Line 159
• Line 160

As described at wordpress-mobile/WordPress-Android#18657 (comment), it has proven difficult to reproduce this crash. It's suspected that there are specific instances with lower performing devices or devices on poor connections that can lead to previousFontMetrics being set as null elsewhere in the code while chooseHeight is running.

Given the difficulty reproducing, it hasn't been possible to get to the heart of this issue. Instead, with this PR, the previous non-null assertions previously used with previousFontMetrics have been replaced with safe calls. With this approach, the functionality should remain the same, while guarding against NullPointerException crashes.

Test

Verify Aztec demo continues to function as expected

1. Load the Aztec demo with this PR's changes applied.
2. Verify that you can add headings with no unexpected side effects.

Verify Gutenberg continues to function as expected

1. Using the installable build available from Address NullPointerException crash in AztecHeadingSpan WordPress-Android#19724, install the app and navigate to the post editor.
2. Add some heading blocks and ensure there are no unexpected side effects when performing tasks. Example tasks to test include splitting the block, applying new font sizes and line heights, etc.

[SiobhyB]

@khaykov, 👋 , I wanted to give you a heads up on this PR as I know Day One uses Aztec for the Android app. Can you take a look to let me know if you spot anything that might cause issues for you? We're hoping to merge this fix in time for a beta in the next day or so.

[fluiddot]

LGTM 🎊 !

Regarding the changes, I forced setting previousFontMetrics to null in the Aztec demo, and seems there are no visible side effects.

I've tested both the Aztec demo app and Gutenberg, the Headings worked as expected 🎊. The only thing I noticed is some unexpected behaviors in the Aztec demo, although they aren't regressions as they can be reproduced in trunk. I can follow-up by opening a GitHub issue.

aztec-android-heading-bugs.mp4

[SiobhyB]

@khaykov, I'm going to go ahead to merge this PR, so we can get it ready for an upcoming beta. Please let me know if there's anything that stands out to you as off, though! I think the changes are fairly safe, but thought it was best to make you aware of them regardless.