Throw error on _startChildApp if child app does not exist

Author: LisaManresa

src/mixins/child-apps.js

@@ -133,6 +133,11 @@ export default {
    */
   startChildApp(appName, options) {
     const childApp = this.getChildApp(appName);
+
+    if (!childApp) {
+      throw new Error(`A child app with the name ${ appName } does not exist.`);
+    }
+
     return this._startChildApp(childApp, options);
   },
 

test/unit/mixins/child-apps.spec.js

@@ -369,6 +369,14 @@ describe('ChildAppMixin', function() {
       expect(this.myChildApp.isRunning()).to.be.true;
     });
 
+    it('should throw an error if childApp does not exist', function() {
+      try {
+        this.myApp.startChildApp('cA0');
+      } catch (e) {
+        expect(e.message).to.equal('A child app with the name cA0 does not exist.');
+      }
+    });
+
     it('should start childApp with options', function() {
       this.myChildApp = this.myApp.startChildApp('cA2', { foo: 'bar' });