fix(engines): enable flexi to better work in nested engines

index.js

@@ -28,10 +28,15 @@ module.exports = {
     this._super.included.apply(this, arguments);
 
     // see: https://github.com/ember-cli/ember-cli/issues/3718
-    if (typeof app.import !== 'function' && app.app) {
+    while (typeof app.import !== 'function' && app.app) {
       app = app.app;
     }
 
+    if (typeof app.import !== 'function') {
+      throw new Error('Flexi is being used within another addon or engine and is' +
+        ' having trouble registering itself to the parent application.');
+    }
+
     var pathBase = this.project.addonPackages.flexi.path;
     compileScssVariables(path.join(pathBase, 'addon/styles'), this.flexiConfig());