problem with "augments" with "when" statements

[dbarrosop]

Hello,
I am trying to load some Openconfig models and I am having problems with some augmentations that have when statements. For instance:

https://github.com/openconfig/public/blob/8527e6426cf136930ff24f60939994949cefc073/release/models/vlan/openconfig-vlan.yang#L436

The error:

self = <yangson.schemanode.SchemaTreeNode object at 0x7f1df2c17898>, stmt = <yangson.statement.Statement object at 0x7f1df2ad2160>, sctx = <yangson.schemadata.SchemaContext object at 0x7f1df2a5c5f8>

    def _augment_stmt(self, stmt: Statement, sctx: SchemaContext) -> None:
        """Handle **augment** statement."""
        if not sctx.schema_data.if_features(stmt, sctx.text_mid):
            return
        path = sctx.schema_data.sni2route(stmt.argument, sctx)
        target = self.get_schema_descendant(path)
        if stmt.find1("when"):
            gr = GroupNode()
            target._add_child(gr)
            target = gr
>       target._handle_substatements(stmt, sctx)
E       AttributeError: 'NoneType' object has no attribute '_handle_substatements'

Removing the when will make it work. Is this a problem with the model or with yangson?

Regards

[dbarrosop]

Just saw #10 and tested it and it seems to solve this issue.