Message: reset() expects parameter 1 to be array, null given

Okay, it's NULL.

I don't understand it, even though $array_container is already declared as an array.
Anyone would, like to help?

Thanks in advance.

PHP is a loosely type language, so type of a variable can be changed if you want.
You should check set_array_container() method to see if $array_container is set correctly.