Clixon fails to load yang with extension

[trentzhou]

I played with the clixon-hello yang. When I added an extension, clixon_backend fails to load it.

module clixon-hello {
    yang-version 1.1;
    namespace "urn:example:hello";
    prefix he;
    revision 2024-09-17 {
        description "bug1: unsupported extension";
    }
    revision 2019-04-17 {
        description
            "Clixon hello world example";
    }
    extension ext1 {
        // this breaks the yang parser
        argument arg1 {}
        description "test extension";
    }
    extension ext2 {
        // this is ok
        argument arg2 ;
        description "test extension";
    }
    container hello{
        container world{
            presence true;
        }
    }
}

This yang is valid by pyang or yanglint from https://github.com/CESNET/libyang.