Ionic2 serve shows blank screen after using formbuilder - ionic-framework

Question

I have a Login page. It was working fine. I changed the login form to use FormBuilder.
There is no error during build process or any reported in the developer tool. But the application shows blank screen when running with ionic serve.
Below is the code changed in login.html.
<form [formGroup]="loginForm" (ngSubmit)="login(loginForm.value)" >
<ion-row>
<ion-col>
<ion-list inset>
<ion-item>
<ion-input type="text" placeholder="Username" formControlName="username" ></ion-input>
</ion-item>
<p *ngIf="username.hasError('required') && username.touched" danger>Username is required</p>
<ion-item>
<ion-input type="password" placeholder="Password" formControlName="password"></ion-input>
</ion-item>
<p *ngIf="password.hasError('required') && password.touched" danger>Password is required</p>
</ion-list>
</ion-col>
</ion-row>
<ion-row>
<ion-col class="signup-col">
<button ion-button class="submit-btn" full type="submit" [disabled]="!loginForm.valid">Login</button>
<button ion-button class="register-btn" block clear (click)="createAccount()">Create New Account</button>
</ion-col>
</ion-row>
</form>
Below is the code changed in login.ts.
import { Component } from '#angular/core';
import { NavController, AlertController, LoadingController, Loading } from 'ionic-angular';
import { FormBuilder, FormGroup, FormControl, Validators } from '#angular/forms';
import { AuthService } from '../../providers/auth-service';
import { RegisterPage } from '../register/register';
import { HomePage } from '../home/home';
#Component({
selector: 'page-login',
templateUrl: 'login.html'
})
export class LoginPage {
loading: Loading;
registerCredentials = {username: '', password: ''};
loginForm: FormGroup;
username: FormControl;
password: FormControl;
constructor(private nav: NavController, private auth: AuthService,
private alertCtrl: AlertController, private loadingCtrl: LoadingController,
public formBuilder: FormBuilder) {
this.loginForm = formBuilder.group({
username: ['', Validators.required],
password: ['', Validators.required],
});
this.username = this.loginForm['username'];
this.password = this.loginForm['password'];
}