fix the key error exception when adding an overwriting key which is defined in the extensions

    if p.scripts is not None:

        p.scripts.before_process(p)

    stored_opts = {k: opts.data[k] for k in p.override_settings.keys() if k in opts.data}

    stored_opts = {k: opts.data[k] if k in opts.data else opts.get_default(k) for k in p.override_settings.keys() if k in opts.data}

    try:

        # if no checkpoint override or the override checkpoint can't be found, remove override entry and load opts checkpoint