How to wrap up functions using NodeIndex in petgraph

Newbie question: I want to use the petgraph rust crate. The graph should be a part of the struct MyGraph. For this, I need to wrap some functions to make them acessible from outside:

use petgraph::graph::DiGraph;
use petgraph::adj::NodeIndex;

#[derive(Default)]
struct MyNode {}

#[derive(Default)]
struct MyGraph {
    g: DiGraph<MyNode,()>
}

impl MyGraph {
    pub fn add_node(&mut self, node: MyNode) -> NodeIndex {
        self.g.add_node(node) // does not compile: return value is u32 != NodeIndex
    }
    pub fn update_edge(&mut self, a: NodeIndex, b: NodeIndex) {
        self.g.update_edge(a, b, ()); // does not compile: a: NodeIndex expected, got u32
    }
}
fn main() {
    let mut my_graph=MyGraph::default();
    let node1=my_graph.add_node(MyNode::default());
    let node2=my_graph.add_node(MyNode::default());
    my_graph.update_edge(node1,node2);
}

The rust compiler however complains the an u32 is returned by the self.g.add_node() function, which is not the case if you call the function directly (and it's documented to return a NodeIndex). The smae problem applies to the self.g.update_edge() function. What would be the correct parameter type to use here???

A return type NodeIndex did not work as well. The (partial) compile error message:

...
error[E0308]: arguments to this function are incorrect
   --> src\main.rs:17:16
    |
17  |         self.g.update_edge(a, b, ());
    |                ^^^^^^^^^^^
    |
note: expected struct `NodeIndex`, found `u32`
   --> src\main.rs:17:28
    |
17  |         self.g.update_edge(a, b, ());
    |                            ^
    = note: expected struct `NodeIndex`
                 found type `u32`
...

What I am doing wrong here?

You're importing petgraph::adj::NodeIndex, which is an alias for u32, but what you need to import is petgraph::graph::NodeIndex, which is a different struct.