FIX: Bug with search

plugin.rb

@@ -16,10 +16,14 @@ after_initialize do
     if Search.respond_to? :advanced_filter
       Search.advanced_filter(/in:kb/) do |posts|
         selected_categories = SiteSetting.knowledge_explorer_categories.split("|")
+        if selected_categories
           categories = Category.where('id IN (?)', selected_categories).pluck(:id)
+        end
 
         selected_tags = SiteSetting.knowledge_explorer_tags.split("|")
+        if selected_tags
           tags = Tag.where('name IN (?)', selected_tags).pluck(:id)
+        end
 
         posts.where('category_id IN (?) OR topics.id IN (SELECT DISTINCT(tt.topic_id) FROM topic_tags tt WHERE tt.tag_id IN (?))', categories, tags)
       end