要解决“Angular - 拦截器始终从localStorage获取accessToken时返回null值”的问题，可以按照以下步骤进行：

1. 确保在登录时将accessToken存储在localStorage中。

2. 创建一个AuthInterceptor拦截器，该拦截器将负责在每个请求上添加accessToken。

import { Injectable } from '@angular/core';
import { HttpInterceptor, HttpRequest, HttpHandler, HttpEvent } from '@angular/common/http';
import { Observable } from 'rxjs';

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  constructor() {}

  intercept(request: HttpRequest, next: HttpHandler): Observable> {
    const accessToken = localStorage.getItem('accessToken');

    if (accessToken) {
      request = request.clone({
        setHeaders: {
          Authorization: `Bearer ${accessToken}`
        }
      });
    }

    return next.handle(request);
  }
}

3. 在app.module.ts文件中将AuthInterceptor添加到providers数组中，并注册为HTTP_INTERCEPTORS。

import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';

import { AppComponent } from './app.component';
import { AuthInterceptor } from './auth.interceptor';

@NgModule({
  imports: [BrowserModule, HttpClientModule],
  declarations: [AppComponent],
  providers: [
    {
      provide: HTTP_INTERCEPTORS,
      useClass: AuthInterceptor,
      multi: true
    }
  ],
  bootstrap: [AppComponent]
})
export class AppModule {}

通过这些步骤，每当发出HTTP请求时，拦截器将从localStorage中获取accessToken并将其添加到请求的头部中。