I am getting error Cannot read property 'length' of undefined

[sedhurabe]

I am using Angular 8 version in my project, Once I install the ngx-dropdown-list I got an error Cannot read property 'length' of undefined.
In My html file

<ngx-dropdown-list (selectionChange)="onChange($event)"
    [items]="option"
    [(selectedValue)]="selectedName">
</ngx-dropdown-list>

In Ts file

import { optionItems } from '../../../config/config';

ngOnInit() {
    this.option = optionItems
  }