fix error "undefined method each for nil" (#60)

app/helpers/application_helper.rb

@@ -22,6 +22,6 @@ def options_with_versions(latest_versions, previous_versions)
     latest_versions.map do |version|
       ancestors = previous_versions.select { |ancestor| ancestor.id.in?(version.ancestor_ids) }
       { ancestors.last&.name || version.name => [[version.name, version.id]] + ancestors.map { |v| [v.name, v.id] } }
-    end.reduce(:merge)
+    end.reduce({}, :merge)
   end
 end