Magento – Magento 2 DOM validation exception when in developer mode

I'm running Magento 2.2.1 and Infortis Ultimo 2.5.1. When I'm in developer mode and I attempt to access a product page on the front end, I get the exception.

Exception #0 (Magento\Framework\Config\Dom\ValidationException): Element 'referenceContainer', attribute 'htmlTag': [facet 'enumeration'] The value '' is not an element of the set {'aside', 'dd', 'div', 'dl', 'fieldset', 'main', 'nav', 'header', 'footer', 'ol', 'p', 'section', 'table', 'tfoot', 'ul'}.
Line: 1159

Element 'referenceContainer', attribute 'htmlTag': '' is not a valid value of the atomic type 'htmlTagType'.
Line: 1159

Element 'referenceContainer', attribute 'htmlClass': [facet 'pattern'] The value '' is not accepted by the pattern '[a-zA-Z][a-zA-Z\d-_](\s[a-zA-Z][a-zA-Z\d-_])*'.
Line: 1159

This is because the htmlTag and htmlClass attributes in app/design/frontend/Infortis/base/Magento_Catalog/layout/catalog_product_view.xml are not being set.

<!-- Reset attributes of main containers -->
<referenceContainer name="product.info.main" htmlTag="" htmlClass="" before="-" />
<referenceContainer name="product.info.media" htmlTag="" htmlClass="" after="product.info.main" />

When switching to production mode the page renders correctly as expected with no errors. I've tried running the following commands:

php -d memory_limit=512M bin/magento cache:clean
php -d memory_limit=512M bin/magento cache:flush
php -d memory_limit=512M bin/magento setup:upgrade
php -d memory_limit=512M bin/magento setup:di:compile
php -d memory_limit=512M bin/magento indexer:reindex

Error is still appearing when in developer mode as described above.