Undefined index notice

Problem/Motivation

Getting the following when using this module to select a slick optionset configuration item from the Slick module:
Notice: Undefined index: handler in config_entity_reference_selection_options_list_alter() (line 20 of modules/contrib/config_entity_reference_selection/config_entity_reference_selection.module).

Proposed resolution

Check that $settings['handler'] exists before trying to access it.