My logs are packed with thousands of these:
Notice: Trying to access array offset on value of type null in template_preprocess_dc_ajax_shopping_cart_teaser() (line 612 of /var/www/html/sites/all/modules/dc_ajax_add_cart/dc_ajax_add_cart.module)
It looks like this is caused when new users visit the site for the first time and have nothing in their cart.
Add nothing to your cart and then load the page as a new session (in a private window or whatever).
Check if that object is null and default to 0 when setting product_price_total? Not sure if this would cause any other problems.
Needs review
2.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.