Update check on features object for cases where not defined

app/assets/javascripts/helpers/startup_css_helper.js

@@ -32,7 +32,7 @@ const handleStartupEvents = () => {
     action();
 -*/
 export const waitForCSSLoaded = (action = () => {}) => {
-  if (!gon.features.startupCss || allLinksLoaded()) {
+  if (!gon?.features?.startupCss || allLinksLoaded()) {
     return new Promise(resolve => {
       action();
       resolve();

spec/frontend/helpers/startup_css_helper_spec.js

@@ -19,6 +19,26 @@ describe('waitForCSSLoaded', () => {
     });
   });
 
+  describe('when gon features is not provided', () => {
+    let originalGon;
+
+    beforeEach(() => {
+      originalGon = window.gon;
+      window.gon = null;
+    });
+
+    afterEach(() => {
+      window.gon = originalGon;
+    });
+
+    it('should invoke the action right away', async () => {
+      const events = waitForCSSLoaded(mockedCallback);
+      await events;
+
+      expect(mockedCallback).toHaveBeenCalledTimes(1);
+    });
+  });
+
   describe('with startup css disabled', () => {
     gon.features = {
       startupCss: false,