I think you are running into the same thing I did here. Your getting the nulls because the information a out the element is only presented after graph execution, which is after the element has been deleted. So, any node that has actual elements will display null while nodes that display text info may still appear to contain the information.
I have switched the python node to a “delete elements node” from Archilab, this seems to work!
However, deleting so many elements at once seems to cause a crash, which is why I looked for python nodes to see if I could delete say a group of 10 at a time.
I have switched back to using a “delete element” python code I found in this thread. This seems to work also. So clearly my issue was within the python I was trying to use.
I think I need to go read this thread and understand it more before coming back to ask how to only delete a group of 10 elements at a time. If I need to do this and get stuck shall I make another thread?