diff --git a/app/assets/javascripts/discourse/app/mapping-router.js b/app/assets/javascripts/discourse/app/mapping-router.js
index d5f1e1874b..cb000cc1f3 100644
--- a/app/assets/javascripts/discourse/app/mapping-router.js
+++ b/app/assets/javascripts/discourse/app/mapping-router.js
@@ -141,10 +141,8 @@ export function mapRoutes() {
     this.route("unknown", { path: "*path" });
   });
 }
-export function teardownRouter(container) {
-  const router = container.lookup("router:main");
-  const constructor = Object.getPrototypeOf(router).constructor;
-  constructor.dslCallbacks.splice(0, constructor.dslCallbacks.length);
+export function teardownRouter(routerClass) {
+  routerClass.dslCallbacks.splice(0, routerClass.dslCallbacks.length);
 }
 
 export function registerRouter(registry) {
diff --git a/app/assets/javascripts/discourse/app/pre-initializers/map-routes.js b/app/assets/javascripts/discourse/app/pre-initializers/map-routes.js
index 85bb8d2ace..19933c1480 100644
--- a/app/assets/javascripts/discourse/app/pre-initializers/map-routes.js
+++ b/app/assets/javascripts/discourse/app/pre-initializers/map-routes.js
@@ -9,8 +9,9 @@ export default {
   after: "inject-discourse-objects",
 
   initialize(container, app) {
-    let router = registerRouter(app);
-    container.registry.register("router:main", router);
+    let routerClass = registerRouter(app);
+    container.registry.register("router:main", routerClass);
+    this.routerClass = routerClass;
 
     if (isLegacyEmber()) {
       // HACK to fix: https://github.com/emberjs/ember.js/issues/10310
@@ -24,7 +25,7 @@ export default {
     }
   },
 
-  teardown(container) {
-    teardownRouter(container);
+  teardown() {
+    teardownRouter(this.routerClass);
   },
 };
diff --git a/app/assets/javascripts/discourse/tests/setup-tests.js b/app/assets/javascripts/discourse/tests/setup-tests.js
index 4c3d16b564..30b36fda8d 100644
--- a/app/assets/javascripts/discourse/tests/setup-tests.js
+++ b/app/assets/javascripts/discourse/tests/setup-tests.js
@@ -73,9 +73,29 @@ function createApplication(config, settings) {
   setApplication(app);
   setResolver(buildResolver("discourse").create({ namespace: app }));
 
+  // Modern Ember only sets up a container when the ApplicationInstance
+  // is booted. We have legacy code which relies on having access to a container
+  // before boot (e.g. during pre-initializers)
+  //
+  // This hack sets up a container early, then stubs the container setup method
+  // so that Ember will use the same container instance when it boots the ApplicationInstance
+  //
+  // Note that this hack is not required in production because we use the default `autoboot` flag,
+  // which triggers the internal `_globalsMode` flag, which sets up an ApplicationInstance immediately when
+  // an Application is initialized (via the `_buildDeprecatedInstance` method).
+  //
+  // In the future, we should move away from relying on the `container` before the ApplicationInstance
+  // is booted, and then remove this hack.
   let container = app.__registry__.container();
   app.__container__ = container;
   setDefaultOwner(container);
+  sinon
+    .stub(Object.getPrototypeOf(app.__registry__), "container")
+    .callsFake((opts) => {
+      container.owner = opts.owner;
+      container.registry = opts.owner.__registry__;
+      return container;
+    });
 
   if (!started) {
     app.start();
@@ -395,6 +415,7 @@ export default function setupTests(config) {
   let settings = resetSettings();
   app = createApplication(config, settings);
   setupTestsCommon(app, app.__container__, config);
+  sinon.restore();
 }
 
 function getUrlParameter(name) {