AFAICT, the issue is that parent
must now be an array or undefined
. I was hit by this bug because I passed 'parent' => null
to acf_register_block_type()
.
https://github.com/WordPress/gutenberg/blob/7506dbbe49367f0641d339473b4416f579bc9376/packages/blocks/src/store/process-block-type.js#L216