We had the same issue after enabling FPC. The following error would occur:
Fatal error: Uncaught Error: Call to a member function getIdentities() on null in /vendor/magento/module-catalog/Block/Category/View.php:177
Not the most helpful error message but after some digging we found that this was being triggered by using the block type
Magento\Catalog\Block\Category\View on our homepage - presumably because there is no category initialised at that point.
This was a coding error by one of our developers as this particular block did not need that type and was fine with just
Magento\Framework\View\Element\Template which is the Magento 2 equivalent of