Stop loading spinner on error of milestone update on issue

app/assets/javascripts/milestone_select.js

@@ -216,6 +216,9 @@ export default class MilestoneSelect {
                   $value.html(milestoneLinkNoneTemplate);
                   return $sidebarCollapsedValue.find('span').text('No');
                 }
+              })
+              .catch(() => {
+                $loading.fadeOut();
               });
           }
         }

changelogs/unreleased/43837-error-handle-in-updating-milestone-on-issue.yml

+---
+title: Stop loading spinner on error of milestone update on issue
+merge_request: 17507
+author: Takuya Noguchi
+type: fixed