[warning] Undefined array key "invalidation" purge_tokens.module:64

Problem/Motivation

While clearing caches or importing configuration via CLI, i get the warning.
[warning] Undefined array key "invalidation" purge_tokens.module:64

Steps to reproduce

• install purge module with some purgers
• use purge_control module to disable purging on a given site - e.g. you have a stage/ local development environment
• run config import or cache clear via drush
• warning will appear

Proposed resolution

• check for existing array keys in purge_tokens module