title: Cannot set property 'title' of undefined in angular 6
body: <p>why I am getting this error
  <strong>Cannot set property 'title' of undefined</strong>
  here is my code
  <a href="https://stackblitz.com/edit/angular-pkd3qr?file=src%2Fapp%2Fapp.component.ts" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-pkd3qr?file=src%2Fapp%2Fapp.component.ts</a></p>
  
  <pre><code>import { Component } from '@angular/core';
  
  
  interface DropDownModel {
    displayName: string;
    value: string;
  }
  
  interface DropdownModelWithtitle {
    title: string;
    dropDownOptions: DropDownModel[];
  }
  
  @Component({
    selector: 'my-app',
    templateUrl: './app.component.html',
    styleUrls: [ './app.component.css' ]
  })
  export class AppComponent  {
    name = 'Angular';
    typeOfProofObj: DropdownModelWithtitle;
    constructor(){
      this.typeOfProofObj.title = "ss";
      this.typeOfProofObj.dropDownOptions = [{displayName:'ss',value:'sss'}];
  
    }
  
  }
  </code></pre>
  
  <p>I am trying to insert the value into my variable, so how to resolve this issue?</p>