TYPO3内容元素：PHP警告：strlen（）期望参数1为字符串，给定数组

I keep getting this error and I am not sure how to solve it. I have searched but none of the answers really helped me.

I went from this code:

$child3 !== null && strlen($child3) > 0 ? array_push($children, (int)$child3) : null;

To this code:

is_array($child3) && count($child3) > 0 ? array_push($children, (int)$child3) : null;

The problem still occurs and I am not sure how to solve it.

I think I still need to check for something else but I'm not sure what.