What?

Guard against null values returned from getBlock.

Why?

Fixes wordpress-mobile/gutenberg-mobile#6128. Mitigate
disruptive crashes caused by accessing attributes on a null value.

How?

In certain scenarios, e.g., when the editor hangs due to poor
performance, the getBlock value unexpectedly returns null. To guard
against a crash in this scenario, we now conditionally access attributes
on the block.

A ideal resolution would be improving editor performance to avoid the
scenario where a race condition causes a crash, but the fact remains
that getBlock can return null values and that we should not
presume that it will not.

Additional details are shared in wordpress-mobile/gutenberg-mobile#6128 (comment).

Testing Instructions

The real world scenario where this occurs is difficult to replicate
consistently, as it appears to depend upon a race condition prevalent during
hangs caused by poor app performance. Therefore, a contrived testing approach
is applying the following diff with and without the proposed changes to ensure that
the component correctly handles null block values.

diff --git a/packages/block-editor/src/components/block-styles/index.native.js b/packages/block-editor/src/components/block-styles/index.native.js
index 40e2947628..b4d4cb2803 100644
--- a/packages/block-editor/src/components/block-styles/index.native.js
+++ b/packages/block-editor/src/components/block-styles/index.native.js
@@ -25,7 +25,7 @@ function BlockStyles( { clientId, url } ) {
 	const selector = ( select ) => {
 		const { getBlock } = select( blockEditorStore );
 		const { getBlockStyles } = select( blocksStore );
-		const block = getBlock( clientId );
+		const block = null;
 		return {
 			styles: getBlockStyles( block?.name ) || EMPTY_ARRAY,
 			className: block?.attributes?.className || '',

1. Insert an Image block.
2. Assign media for the block.
3. Open the block settings.
4. Verify the app does not throw an error or crash.

Testing Instructions for Keyboard

n/a

Screenshots or screencast

It is worth noting that guarding against the crash results in an unexpected block controls state where the block styles are missing. Hypothetically, the block styles would become visible once the block value is no longer null. This is unavoidable until the editor performance is addressed and the race condition is avoided.

Non-null value	Null value