无法为变量赋值null

Question

到目前为止，我对班上的各种错误感到困惑。

我正在创建一个返回一些数据的服务（它很快会来自数据库，因此将其设置为可观察的数据）。

第一个错误是@Injectable存在一些问题

第二个错误是我无法将null赋给变量？

我尝试从角度教程中获取此信息：https://angular.io/tutorial/toh-pt4

的package.json：

{
  "name": "SpaceGame.WebUI",
  "private": true,
  "version": "0.0.0",
  "scripts": {
    "test": "karma start ClientApp/test/karma.conf.js"
  },
  "devDependencies": {
    "@angular/animations": "4.2.5",
    "@angular/common": "4.2.5",
    "@angular/compiler": "4.2.5",
    "@angular/compiler-cli": "4.2.5",
    "@angular/core": "4.2.5",
    "@angular/forms": "4.2.5",
    "@angular/http": "4.2.5",
    "@angular/platform-browser": "4.2.5",
    "@angular/platform-browser-dynamic": "4.2.5",
    "@angular/platform-server": "4.2.5",
    "@angular/router": "4.2.5",
    "@ngtools/webpack": "1.5.0",
    "@types/chai": "4.0.1",
    "@types/jasmine": "2.5.53",
    "@types/webpack-env": "1.13.0",
    "angular2-router-loader": "0.3.5",
    "angular2-template-loader": "0.6.2",
    "aspnet-prerendering": "^3.0.1",
    "aspnet-webpack": "^2.0.1",
    "awesome-typescript-loader": "3.2.1",
    "bootstrap": "3.3.7",
    "chai": "4.0.2",
    "css": "2.2.1",
    "css-loader": "0.28.4",
    "es6-shim": "0.35.3",
    "event-source-polyfill": "0.0.9",
    "expose-loader": "0.7.3",
    "extract-text-webpack-plugin": "2.1.2",
    "file-loader": "0.11.2",
    "html-loader": "0.4.5",
    "isomorphic-fetch": "2.2.1",
    "jasmine-core": "2.6.4",
    "jquery": "3.2.1",
    "json-loader": "0.5.4",
    "karma": "1.7.0",
    "karma-chai": "0.1.0",
    "karma-chrome-launcher": "2.2.0",
    "karma-cli": "1.0.1",
    "karma-jasmine": "1.1.0",
    "karma-webpack": "2.0.3",
    "preboot": "4.5.2",
    "raw-loader": "0.5.1",
    "reflect-metadata": "0.1.10",
    "rxjs": "5.4.2",
    "style-loader": "0.18.2",
    "to-string-loader": "1.1.5",
    "typescript": "2.4.1",
    "url-loader": "0.5.9",
    "webpack": "2.5.1",
    "webpack-hot-middleware": "2.18.2",
    "webpack-merge": "4.1.0",
    "zone.js": "0.8.12"
  }
}