Console error : No value accessor for form controll with name:

Question

I have added ng-select in my project for multiselect-searchable-grouping dropdown. I have followed the steps from https://ng-select.github.io/ng-select#/grouping, but unfortunately getting: ERROR Error: No value accessor for form control with name: 'optProject'

I am using angular 6

Please see my code as below: I have added NgSelectModule to module.ts

html

<ng-select formControlName="optProject"
            [items]="projects"
            bindLabel="title"
            bindValue="id"
            groupBy="subprojects"
            [multiple]="true"
            [(ngModel)]="selectedProjects">
    <ng-template ng-optgroup-tmp let-item="item">
        {{item.title}}
    </ng-template>
    <ng-template ng-option-tmp let-item="item">
        {{item.title}}
    </ng-template>
</ng-select>

component.ts

import { FormGroup, FormControl} from '@angular/forms';
.
.
.

public selectedProjects;
public projects;


ngOnInit() {
    this.selectedProjects = [];
    this.projects = [
        {
            id: 'p1',
            title: 'Project A',
            subprojects: [
                { title: 'Subproject 1 of A', id: 's1p1' },
                { title: 'Subproject 2 of A', id: 's2p1' },
            ]
        },
        {
            id: 'p2',
            title: 'Project B',
            subprojects: [
                { title: 'Subproject 1 of B', id: 's1p2' },
                { title: 'Subproject 2 of B', id: 's2p2' },
            ]
        }
    ]
}

projectForm = new FormGroup({
    optProject: new FormControl(null),
});

While searching on Google, I found that this kind of error occurs while adding formCorntrolName to labels/div instead of input controls. But is the case here, with ng-select ? Whats wrong with my code...! Thanks in advance...

Just to add, I have added ngDefaultControl to ng-select, because I found: Third party controls require a ControlValueAccessor to function with angular forms (What is ngDefaultControl in Angular?), but still no luck!!! The error goes but no content display within <ng-select>