Cannot read property 'required' of null for radio button?

Question

Angular

There is this code:

<ng-container *ngIf="fieldDefinition.fieldtype == 'radio'">
            <div class="col-md-12">
                <label>{{ fieldDefinition.title }}</label>
                <ng-container *ngFor="let radio of fieldDefinition?.datasource?.value">
                    <div class="form-check form-check-radio">
                        <label class="form-check-label">
                            <input
                                class="form-check-input"
                                type="radio"
                                name="radio_{{ id }}"
                                #ref="ngModel"
                                id="{{ id }}"
                                (change)="handleRadioChange(fieldDefinition)"
                                [checked]="radio.id == fieldDefinition.fieldvalue"
                                [value]="radio.id"
                                [(ngModel)]="fieldDefinition.fieldvalue"
                            />
                            {{ radio.value }}
                            <span class="circle">
                                <span class="check"></span>
                            </span>
                        </label>
                    </div>
                    <div *ngIf="ref.errors.required">Поле обязательно для заполнения</div>
                </ng-container>
            </div>
        </ng-container>

Why am I getting this error

Cannot read property 'required'