Python infix to postfix conversion

In Python, I've begun to solve Data Structure difficulties. I'm converting from infix to postfix, however, I'm having trouble debugging a mistake in my software. The last retu statement, when I do the join operation, contains NONE input.
When I started debugging, I saw that something is incorrect in this section of code following the first push operation (for *). After that, whenever I call pop(), it retus NONE instead of *. Could someone please point out what is wrong here?
Python infix to postfix conversion