Fix JS error when discussion has no diff_file

This can be caused by opening a merge request to a note
that is no in a diff, but just in the overview tab

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/230900

app/assets/javascripts/diffs/store/actions.js

@@ -748,9 +748,11 @@ export const setCurrentDiffFileIdFromNote = ({ commit, rootGetters }, noteId) =>
 
   if (!note) return;
 
-  const fileHash = rootGetters.getDiscussion(note.discussion_id).diff_file.file_hash;
+  const fileHash = rootGetters.getDiscussion(note.discussion_id).diff_file?.file_hash;
 
+  if (fileHash) {
     commit(types.UPDATE_CURRENT_DIFF_FILE_ID, fileHash);
+  }
 };
 
 export const navigateToDiffFileIndex = ({ commit, state }, index) => {

spec/frontend/diffs/store/actions_spec.js

@@ -1603,6 +1603,18 @@ describe('DiffsStoreActions', () => {
 
       expect(commit).toHaveBeenCalledWith(types.UPDATE_CURRENT_DIFF_FILE_ID, '123');
     });
+
+    it('does not commit UPDATE_CURRENT_DIFF_FILE_ID when discussion has no diff_file', () => {
+      const commit = jest.fn();
+      const rootGetters = {
+        getDiscussion: () => ({ id: '1' }),
+        notesById: { '1': { discussion_id: '2' } },
+      };
+
+      setCurrentDiffFileIdFromNote({ commit, rootGetters }, '1');
+
+      expect(commit).not.toHaveBeenCalled();
+    });
   });
 
   describe('navigateToDiffFileIndex', () => {