Get item at the same index from multiple lists

Hello, I`m trying to set the Material to all of the created forms, but an error occurred. The warning says:

“Warning: List.GetItemAtIndex operation failed.
Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index”

Any suggestions where the problem is?

Use the correct Level to get the Item #4 in Items #1-3 since you have nested lists.