htmlspecialchars() expects parameter 1 to be string, array given in check_plain() (line 1571 of bootstrap.inc)

context_reaction::fetch_from_context returns an empty array if the plugin isn't set for the context. In context_reaction_template_suggestions::options_form(), this is used as the field's default value, causing an array to be passed to check_plain.

I'll have a patch for this shortly.