fix(ember): use _backburner if it exists (#4603)

Author: pbernery

packages/ember/addon/instance-initializers/sentry-performance.ts

@@ -2,6 +2,7 @@ import ApplicationInstance from '@ember/application/instance';
 import Ember from 'ember';
 import { run, _backburner, scheduleOnce } from '@ember/runloop';
 import * as Sentry from '@sentry/browser';
+import { ExtendedBackburner } from "@sentry/ember/runloop";
 import { Span, Transaction, Integration } from '@sentry/types';
 import { EmberRunQueues } from '@ember/runloop/-private/types';
 import { getActiveTransaction } from '..';
@@ -28,11 +29,19 @@ export function initialize(appInstance: ApplicationInstance): void {
   }
 }
 
-function getBackburner() {
+function getBackburner(): Pick<ExtendedBackburner, "on" | "off"> {
+  if (_backburner) {
+    return _backburner;
+  }
+
   if (run.backburner) {
     return run.backburner;
   }
-  return _backburner;
+
+  return {
+    on() {},
+    off() {}
+  }
 }
 
 function getTransitionInformation(transition: any, router: any) {