Object is possibly NULL

Question

Hi

  In below code i an getting above error on below line

<div *ngIf="form.get('doj').hasError('dateComparison')">
                    <small>Date of joining must be after date of birth.</small>
                </div>

<mat-form-field>
                <mat-label>Choose a date</mat-label>
                <input matInput [matDatepicker]="picker" formControlName="dob">
                <mat-hint>MM/DD/YYYY</mat-hint>
                <mat-datepicker-toggle matIconSuffix [for]="picker"></mat-datepicker-toggle>
                <mat-datepicker #picker></mat-datepicker>
            </mat-form-field>
            <mat-form-field>
                <mat-label>Choose a date</mat-label>
                <input matInput [matDatepicker]="picker" appDateComparison="dob" formControlName="doj">
                <mat-hint>MM/DD/YYYY</mat-hint>
                <mat-datepicker-toggle matIconSuffix [for]="picker"></mat-datepicker-toggle>
                <mat-datepicker #picker></mat-datepicker>
                <div *ngIf="form.get('doj').hasError('dateComparison')">
                    <small>Date of joining must be after date of birth.</small>
                </div>
            </mat-form-field>

Thanks