Gutenberg: PHP notice when adding a Post author block

[carolinan]

Describe the bug
I switched from Gutenberg version 8.0.0 to master, and this PHP notice appeared.

The notice appears in:

• The standard block editor when a post author block is added.
• On the front when viewing a single.html full site editing template.

-It does _not_ appear in the site editor when viewing the single template.

Notice: Trying to get property 'attributes' of non-object in /var/www/html/wp-content/plugins/gutenberg/build/block-library/blocks/post-author.php on line 96

To reproduce
Steps to reproduce the behavior:

1. Enable full site editing
2. Create a new post
3. Add a post author block and save
4. See error

Note:
The content/ markup of my single.html file is:

<!-- wp:post-title {"className":"post-title"} /-->

<!-- wp:post-date /-->

<!-- wp:post-author /-->

<!-- wp:post-comments /-->

<!-- wp:post-comments-form /-->

Expected behavior
I expect the post author data to display correctly.

Editor version (please complete the following information):
WordPress version: 5.4.1
Gutenberg: Master, latest commit was: b73bb6d

Desktop (please complete the following information):
Windows 10
Chrome version: 81.0.4044.129

[youknowriad]

cc @epiqueras @aduth in case this rings any bells. (Maybe block context)

[carolinan]

Note that the single.html _does not have a post content block_... I will keep testing

[aduth]

Looks like it's from #19894 (cc @draganescu)

https://github.com/WordPress/gutenberg/pull/19894/files#diff-585397093c781537eba985402a000b3aR96

It's quite likely there was a confusion stemming from the introduction and subsequent removal of a block object as first argument of render_callback (#21467, #21868, #21921). Currently, the first argument of render_callback cannot be referenced as an object in this way.

It's my hope to merge #21925 today, at which point this could be fixed by addressing $block object as the _third_ argument of render_callback.

[aduth]

Fix at #22114

[carolinan]

God sticky mobile fingers