Fix error thrown with missing note fragment in DOM

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32888

Reproduction:

 1. Visit /namespace/project/merge_requests/x/diffs#note_1234
 1. When `#note_1234` isn't in the diff, an error is thrown

app/assets/javascripts/merge_request_tabs.js

@@ -285,7 +285,7 @@ import BlobForkSuggestion from './blob/blob_fork_suggestion';
           // Similar to `toggler_behavior` in the discussion tab
           const hash = window.gl.utils.getLocationHash();
           const anchor = hash && $container.find(`[id="${hash}"]`);
-          if (anchor) {
+          if (anchor && anchor.length > 0) {
             const notesContent = anchor.closest('.notes_content');
             const lineType = notesContent.hasClass('new') ? 'new' : 'old';
             notes.toggleDiffNote({

spec/javascripts/merge_request_tabs_spec.js

 /* eslint-disable no-var, comma-dangle, object-shorthand */
+/* global Notes */
 import '~/merge_request_tabs';
 import '~/commit/pipelines/pipelines_bundle';
@@ -7,6 +8,7 @@ import '~/lib/utils/common_utils';
 import '~/diff';
 import '~/single_file_diff';
 import '~/files_comment_button';
+import '~/notes';
 import 'vendor/jquery.scrollTo';
 (function () {
@@ -29,7 +31,7 @@ import 'vendor/jquery.scrollTo';
       };
       $.extend(stubLocation, defaults, stubs || {});
     };
-    preloadFixtures('merge_requests/merge_request_with_task_list.html.raw');
+    preloadFixtures('merge_requests/merge_request_with_task_list.html.raw', 'merge_requests/diff_comment.html.raw');
     beforeEach(function () {
       this.class = new gl.MergeRequestTabs({ stubLocation: stubLocation });
@@ -286,8 +288,49 @@ import 'vendor/jquery.scrollTo';
         spyOn($, 'ajax').and.callFake(function (options) {
           expect(options.url).toEqual('/foo/bar/merge_requests/1/diffs.json');
         });
+
         this.class.loadDiff('/foo/bar/merge_requests/1/diffs');
       });
+
+      describe('expands and scrolls to linked fragment hash #note_xxx', () => {
+        beforeEach(() => {
+          loadFixtures('merge_requests/diff_comment.html.raw');
+          spyOn(window.gl.utils, 'getPagePath').and.returnValue('merge_requests');
+          window.notes = new Notes('', []);
+          spyOn(window.notes, 'toggleDiffNote').and.callThrough()
+        });
+
+        afterEach(() => {
+          delete window.notes;
+        });
+
+        it('opens note associated with fragment hash', function () {
+          const noteId = 'note_1';
+          spyOn(window.gl.utils, 'getLocationHash').and.returnValue(noteId);
+          spyOn($, 'ajax').and.callFake(function (options) {
+            options.success({ html: `<div id="${noteId}">foo</div>` });
+          });
+
+          this.class.loadDiff('/foo/bar/merge_requests/1/diffs');
+
+          expect(window.notes.toggleDiffNote).toHaveBeenCalledWith({
+            target: jasmine.any(Object),
+            lineType: 'old',
+            forceShow: true,
+          });
+        });
+
+        it('does not throw an error when loading with a fragment hash that does not exist', function () {
+          spyOn(window.gl.utils, 'getLocationHash').and.returnValue('note_something-that-does-not-exist');
+          spyOn($, 'ajax').and.callFake(function (options) {
+            options.success({ html: '' });
+          });
+
+          this.class.loadDiff('/foo/bar/merge_requests/1/diffs');
+
+          expect(window.notes.toggleDiffNote).not.toHaveBeenCalled();
+        });
+      });
     });
   });
 }).call(window);