Saving changes to block style causes Drupal error

Problem/Motivation

Clicking "Save Block" causes Drupal website error

Steps to reproduce

1. Configure Block
2. Click on "Save Block;" - can make changes or not - doesn't matter
3. Following error message:

The website encountered an unexpected error. Please try again later.
Error: Call to undefined method Drupal::entityManager() in _block_styles_form_submit() (line 132 of modules/block_styles/block_styles.module)

Proposed resolution

None

Remaining tasks

User interface changes

API changes

Data model changes