[formGroupName]="i"应该在*ngFor内.在这种情况下，i变量将具有非undefined值

[formGroupName]="i" should be inside of *ngFor. In this case i variable will have non undefined value

<div *ngFor="let media of cardForm.controls.media.controls; let i=index">
  <span>Media {{i + 1}}</span>
  <span *ngIf="cardForm.controls.media.controls.length > 1" (click)="removeMedia(i)"></span>
  <div [formGroupName]="i">
    <div>
      <label>Url</label>
      <md-input-container class="mdcontainer">
        <input mdInput placeholder="Media Url" type="text" formControlName="raw">
      </md-input-container>
    </div>
  </div>
</div>

柱塞示例

Plunker Example