Use new get_active_plugins everywhere, and prevent warnings when no plugins are active

@georgestephanis I liked what you've done in e86d0ab4aeadde96cc3c6026867b8b1252bbdf2d and 8b8a32f59c50caacda67334f05ebe093ad372210!

Could we now use `get_active_plugins` everywhere? I also added #353 to this PR, since it's related.