Restore compatibility with Ember Engines previously fixed in #315

addon/instance-initializers/add-modals-container.js

@@ -1,4 +1,7 @@
 import { getDestinationElementIdFromConfig } from 'ember-modal-dialog/utils/config-utils';
+import Application, { getOwner } from '@ember/application';
+import Engine from '@ember/engine';
+import { assert } from '@ember/debug';
 
 let hasDOM = typeof document !== 'undefined';
 
@@ -24,5 +27,17 @@ export default function (instance) {
   let config = instance.resolveRegistration('config:environment');
   let modalContainerElId = getDestinationElementIdFromConfig(config);
 
-  appendContainerElement(instance.rootElement, modalContainerElId);
+  let app;
+  if (instance instanceof Application) {
+    app = instance;
+  } else if (instance instanceof Engine) {
+    // As there is only a single `Router` across the whole app, which is owned
+    // by the root `Application`, this reliably finds the root `Application`
+    // from an`Engine`.
+    app = getOwner(getOwner(instance).lookup('service:router'));
+  } else {
+    assert(`Could not find the root Application for '${instance}'.`);
+  }
+
+  appendContainerElement(app.rootElement, modalContainerElId);
 }