Boa noite!

Após fazer o exercício, tive o erro abaixo no console do navegador e nada foi renderizado:

GET http://localhost:4200/null 404 (Not Found)

Segue códigos que abaixo.

photo.component.ts:

import { Component } from '@angular/core';

@Component({
    selector: "ap-photo",
    templateUrl: "photo.component.html"
})
export class PhotoComponent{

  description = "World of Warcraft";
  url = "https://vignette.wikia.nocookie.net/wowadventure/images/3/35/World_of_Warcraft_Icon_by_Onix9.png/revision/latest?cb=20131111210301";

}

Photo.component.html:

<img class="img-thumbnail" [src]="url" [alt]="description">

app.component.ts:

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { PhotoComponent } from './photo/photo.component';

@NgModule({
  declarations: [
    AppComponent,
    PhotoComponent
  ],
  imports: [
    BrowserModule
  ],
  providers: [],
  bootstrap: [AppComponent]
})
export class AppModule { }

Estou utilizando as seguintes configurações de software:

Linux Mint 19.1

Visual Studio Code 1.30.2

Angular CLI 7.2.1

Node.js 11.6.0