Unable to place GAMW ad blocks - AJAX error likely caused by PHP error

Problem/Motivation

Attempting to place an ad block fails (results in an error that can be seen in console).

AJAX error:

Uncaught
Object { message: "\nAn AJAX HTTP error occurred.\nHTTP Result Code: 200\nDebugging information follows.\nPath: /admin/structure/block/add/gamw/example?region=sidebar_first\nStatusText: OK\nResponseText: ", name: "AjaxError", stack: "@https://dev.example.com/sites/default/files/js/js_LS4djmx8InBABTJ-bazRpr...\n@https://dev.example.com/sites/default/files/js/js_LS4djmx8InBABTJ-bazRpr...\n" }

Suspiciously coincidentally timed PHP error:

TypeError: array_keys(): Argument #1 ($array) must be of type array, null given in array_keys() (line 116 of web/modules/contrib/gamw/src/Plugin/Block/Gamw.php

Steps to reproduce

1. Configure GAMW, uploading ad units CSV file with the following columns: #Id, Parent Id, Code, Name, Sizes, Description, Enabled for AdSense, Placements, Target Window, Labels (where Description, Placement, and Labels are blank for each row)
2. Go to block admin page and attempt to place a GAMW Advertisement block.

Proposed resolution

Clearly this is not happening to everyone; is there a step i am missing?

Remaining tasks