«Notice: Undefined index: class in lib/internal/Magento/Framework/View/Layout/Generator/Block.php on line 213»

Magento 2 does not check whether the array key class exists in the array:

I my case the cause of the failure was my wrong referenceContainer tag instead of referenceBlock.

I have same issue, anybody help!!!
Notice: Undefined index: class in D:\xampp\htdocs\mag\mag_check\vendor\magento\framework\View\Layout\Generator\Block.php on line 213

You should read carefully the last sentence of the acrticle.