while printing the postorder of the tree, i kept getting None as the last value I have no idea which part i did wrong

def postorder(tree): if tree :     postorder(tree.get_left_subtree())     postorder(tree.get_right_subtree())     print(tree.get_value(),end = "")  def buildTree(inorder, preorder):     if len(inorder) == 0:         return None     if len(inorder) == 1:         return ListBinaryTree(inorder[0])     root = ListBinaryTree(preorder[0])     index = inorder.index(preorder[0])     root.left = root.insert_tree_left(buildTree(inorder[0:index], preorder[1: index + 1]))     root.right = root.insert_tree_right(buildTree(inorder[index+1 : len(inorder)], preorder[index + 1 : len(preorder)-1]))     return root