Unwrap on mesh with no index buffer

[JCapucho]

Bevy version

I tested both the 0.4 version and commit 4a08370 on master

Operating system & version

Arch linux using the 5.10 kernel and latest nvidia drivers

What you did

Created a mesh with only the positions attribute

use bevy::{prelude::*, render::pipeline::PrimitiveTopology};

fn main() {
    App::build()
        .add_plugins(DefaultPlugins)
        .add_startup_system(setup.system())
        .run();
}

fn setup(commands: &mut Commands, mut meshes: ResMut<Assets<Mesh>>) {
    let positions = vec![[0., 0., 0.], [0., 0., 1.], [1., 0., 0.]];
    let mut mesh = Mesh::new(PrimitiveTopology::TriangleList);
    mesh.set_attribute(Mesh::ATTRIBUTE_POSITION, positions);

    commands.spawn(MeshBundle {
        mesh: meshes.add(mesh),
        ..Default::default()
    });
}

What you expected to happen

The program running normally or crashing because of missing attributes

What actually happened

The program crashed because it tried to unwrap the value returned by mesh.get_index_buffer_bytes() and it was None because the mesh didn't have a index buffer

Additional information
The unwrap occurs in bevy_render/src/mesh/mesh.rs:395:48