Error: Cannot access offset of type string on string

Problem/Motivation

When updating multiple nodes I get an error sometimes:
[error] TypeError: Cannot access offset of type string on string in Drupal\menu_to_taxonomy_assign\AutoAssignedTermReferenceItemList->Drupal\menu_to_taxonomy_assign\{closure}() (line 59 of (...)/menu_to_taxonomy/menu_to_taxonomy_assign/src/AutoAssignedTermReferenceItemList.php)

Proposed resolution

There's a wrong array build. Auto-assign term array should contain a target_id key