Describe the bug
When a reusable block has a reusable block embedded (containing the same ID) it throws a 500 error and the 'Reusable' tab disappears in WP 5.3.

To reproduce
1 Go to wp-admin/edit.php?post_type=wp_block&paged=1

2. Create a new reusable block. For this test, I am calling it 'Reusable Block Test'. You can call it anything you'd like. Publish and save, note the post ID from the URL.

3. Embed a new reusable block into the one you just created. Pick a random block.

4. Switch the view to 'Code Editor' and enter the ID from step Add/local build #2. Update the post.

5. You should get an error : 'Updating failed. or Updating failed. Error message: The response is not a valid JSON response. (note that it did update)

6. Go to Posts -> Add New. Open up the "Network" tab in Google Chrome. Clear anything there to get a fresh list.

7. Add a new block. You should get a 500 error and the "Reusable" tab is missing.

Request URL: */wp-json/wp/v2/blocks?per_page=100&_locale=user
Request Method: GET
Status Code: 500
Remote Address: 127.0.0.1:8080
Referrer Policy: strict-origin-when-cross-origin

8. Editing the reusable block from step 2 results in a frontend error:
   "There has been a critical error on your website. Please check your site admin email inbox for instructions"

Expected behavior
Adding a block allows you to add previously created Reusable Blocks by using the "Reusable" tab

Desktop (please complete the following information):

• OS: [e.g. iOS] Windows 10
• Browser [e.g. chrome, safari]: Chrome
• Version [e.g. 22] 78.0.3904.108

Additional context
WP Version: 5.3