ОШИБКА Ошибка: formGroup ожидает экземпляр FormGroup. Пожалуйста, передайте один в

follow this code
demo.html
<form [formGroup]="registerForm" (ngSubmit)="onSubmit()">

                    <div class="form-group">
                        <label>Email</label>
                        <input type="text" formControlName="email" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.email.errors }" />
                        <div *ngIf="submitted && f.email.errors" class="invalid-feedback">
                            <div *ngIf="f.email.errors.required">Email is required</div>
                            <div *ngIf="f.email.errors.email">Email must be a valid email address</div>
                        </div>
                    </div>
                    <div class="form-group">
                        <label>Password</label>
                        <input type="password" formControlName="password" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.password.errors }" />
                        <div *ngIf="submitted && f.password.errors" class="invalid-feedback">
                            <div *ngIf="f.password.errors.required">Password is required</div>
                            <div *ngIf="f.password.errors.minlength">Password must be at least 6 characters</div>
                        </div>
                    </div>
                </form>
-------------------------------------------------------
demo.js

<form [formGroup]="registerForm" (ngSubmit)="onSubmit()">

<div class="form-group">
<label>Email</label>
<input type="text" formControlName="email" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.email.errors }" />
<div *ngIf="submitted && f.email.errors" class="invalid-feedback">
<div *ngIf="f.email.errors.required">Email is required</div>
<div *ngIf="f.email.errors.email">Email must be a valid email address</div>
</div>
</div>
<div class="form-group">
<label>Password</label>
<input type="password" formControlName="password" class="form-control" [ngClass]="{ 'is-invalid': submitted && f.password.errors }" />
<div *ngIf="submitted && f.password.errors" class="invalid-feedback">
<div *ngIf="f.password.errors.required">Password is required</div>
<div *ngIf="f.password.errors.minlength">Password must be at least 6 characters</div>
</div>
</div>
</form>