Fix uncaught TypeError in Columns block

[nerrad]

Description

The following error was discovered for the columns block:

Uncaught (in promise) TypeError: Cannot read property 'innerBlocks' of null.

To Reproduce:

1. Create a columns block with two columns with content.
2. Delete the block in each column.
3. Delete the column container (triggers the error in the console).

The fix is just to ensure we're accessing innerBlocksproperty on a valid block object.

How has this been tested?

• Ensure unit tests/e2e tests don't fail.
• Ran through reproducible steps on this branch and ensure error doesn't appear again.

Types of changes

This is a non-breaking bug fix.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[gziolo]

Code change look good, I haven't tested it.

[gziolo]

Out of curiosity, I'm wondering if the following would work as well:

const { innerBlocks = DEFAULT_EMPTY_ARRAY } = getBlocksByClientId( clientId )[ 0 ];

[nerrad]

That won't work if the array returned by getBlocksByClientId is empty. It'll result in something like:

"Can't destructure property 'innerBlocks' of 'undefined' or 'null'"