TypeError: Unsupported operand types: null + array in _tableofcontents_replace_headers

Problem/Motivation

Warning: Undefined array key "back_to_top" in
theme_tableofcontents_back_to_top() (line 234 oftableofcontents/tableofcontents.themes.inc). .

And
TypeError: Unsupported operand types: null + array in _tableofcontents_replace_headers() (line 54 of tableofcontents/tableofcontents.inc).

Steps to reproduce

Cron + searchapi indexing the entire rendered full page seemed to do the trick

Proposed resolution

Check if the tableofcontents_toc() has got anything before it used.

Remaining tasks

User interface changes

API changes

Data model changes