Reading site.yaml or page frontmatter as a variable in plugin

I have a config in my site.yaml file:

git_changelog: https://domain.com/api/v3/repos/repo/releases

and I want to read it in as a variable in my plugin. I tried to use:

$config = $grav['config']->get('site.git_changelog');

but get an error.

Listing the error would help, but this works in 1.4.1: $this->grav['config']->value('site.title'). Or, if you need to return it in a static function, for example when called by a blueprint: Grav::instance()['config']->value('site.title').