Logic Exception with preview button for block-comment

Problem/Motivation

Exception thrown when previewing a comment on a block content entity with content translation enabled.

Steps to reproduce:

1. Install the standard profile
2. Install the content translation module
3. Create a comment type and target entity is *block*
4. Create a custom block-library with a comment field,
5. Now create a simple block for the new existing block-type in a custom block library
6. Place the block on a region e.g. side-bar-first,
7. Now try to write something in a comment, click on *preview* showing in a blank page *The website encountered an unexpected error. Please try again later.*
8. Once check the log it provides the Logic Exception, seems during this try to *serialize* the db connection object which is being prevented from __sleep

Proposed resolution

Prevent database from being serialised.

Remaining tasks

Write tests

User interface changes

None

API changes

None

Data model changes

None