How to display dialog box when user enter wrong input

Ionic Framework ionic-v3
1

I want to display dialog box when validation fails.

currently i used formBuilder for validate the form but using this i couldn’t know how to display error message in dialogbox.

code that i used

constructor(private navController: NavController, private fb: FormBuilder,private service: WebService) {
        this.authForm = fb.group({  
            'username': ['', Validators.compose([Validators.required, Validators.minLength(3)])],
            'password': ['', Validators.compose([Validators.required, Validators.minLength(3)])]
        });
}

html code

<ion-content class="home" padding>
    <form [ngFormModel]="authForm" (ngSubmit)="onSubmit(authForm.value)">
        <ion-item [class.error]="!username.valid && username.touched">
            <ion-label floating>Username</ion-label>
            <ion-input type="text" value={{userValue}} [ngFormControl]="username"></ion-input>         
        </ion-item>
        <div *ngIf="username.hasError('required') && username.touched" 
            class="error-box">* Username is required!</div>
        <div *ngIf="username.hasError('minlength') && username.touched" 
            class="error-box">* Minimum username length is !</div>            
                             
        <ion-item [class.error]="!password.valid && password.touched">
            <ion-label floating>Password</ion-label>
            <ion-input type="password" value = {{pwdValue}} [ngFormControl]="password" ></ion-input>
        </ion-item>
        <div *ngIf="password.hasError('required') && password.touched" 
            class="error-box">* Password is required</div>    
        <div *ngIf="password.hasError('minlength') && password.touched" 
            class="error-box">* Minimum password length is 3!</div>        
        <br/><br/> 
        <ion-item>
            <ion-label>Remember Me</ion-label>
            <ion-checkbox [(ng-model)]='isChecked' ng-change='checkedOrNot(isChecked)' checked ={{isChecked}}></ion-checkbox>
        </ion-item>
        <br/><br/> 
        <button type="submit" class="custom-button" [disabled]="!authForm.valid" block>Submit</button>
    </form>
</ion-content>