Error occurs when trying to upload image that dos not have
$_SERVER['HTTP_CONTENT_RANGE'] set..

PHP Warning:  Trying to access array offset on value of type null in
/var/www/html/blah-dev/modules/FilePicker/lib/class.jquery_upload_handler.php on
line 305

Fix, replace line 305:
            $total_file_size = $content_range[3];
With:
            $total_file_size = $content_range ? $content_range[3] : null;

(Same as fix already applied in
/FileManager/lib/class.jquery_upload_handler.php)