How to fix “Cannot read property 'icons' of null” error?
To anyone who is running into this issue:

 * Defines the "inline_comment" plugin.
 *
 * @CKEditorPlugin(
 *   id = "machine_name_that_must_match_with_icons",
 *   label = @Translation("CKEditor Inline Comment")
 * )
 */


and 

getButtons() must return the same ID.

Annotation is new to me and I am sure it's not new to anyone else who are Drupal 8 developers but it might help someone who is new/still learning about how Drupal 8 works.

https://www.drupal.org/docs/8/api/plugin-api/annotations-based-plugins