backend/app/controllers/tag_versions_controller.rb

@@ -69,8 +69,11 @@ class TagVersionsController < ApplicationController
 
       parent_tags =
           build_version_values(cur_parent_tag_ids, prev_parent_tag_ids, key: :tag_id)
-          .map do |h|
-            { tag: parent_json.fetch(h[:tag_id]),
+          .filter_map do |h|
+            parent = parent_json[h[:tag_id]]
+            next unless parent
+
+            { tag: parent,
               type: h[:type] }
           end