Undefined index: simple_sku Magento 1.9

I am writing code in magento 1.9 checkout and when adding a new product to cart it generates the error in the log, and my code seems to be correct.

Error message that appears in the log.

The problem is that I don’t understand the reason for the error if the code seems to be correct.
I’m trying to get a code a value from the following array.

My code is the following:

Why can’t I call this value using “$_customOptions[‘simple_sku’]” ?