nullinjectorerror no provider for matdialog unit test code example

Example: NullInjectorError: No provider for MatDialogRef!

import {DialogService} from './services/dialog.service';
  import {MatDialogModule, MatDialogRef} from '@angular/material/dialog';
  ...
  imports: [
    ...
    MatDialogModule
  ],
  providers: [
     {
       provide: MatDialogRef,
       useValue: {}
     },
     DialogService
  ],
  ...