title: mat-button is not a known element
body: <p>I don't get it. I think I imported all necessary module, but it keeps telling me: </p>
  
  <pre><code>'mat-button' is not a known element:
  </code></pre>
  
  <p>app.module.ts:</p>
  
  <pre><code>import { NgModule } from '@angular/core';
  import { FormsModule } from '@angular/forms';
  import { MatButtonModule } from '@angular/material/button';
  import { BrowserModule } from '@angular/platform-browser';
  import { BrowserAnimationsModule } from '@angular/platform-browser/animations';
  
  import { AppComponent } from './app.component';
  
  @NgModule({
    imports: [
      BrowserModule,
      FormsModule,
      BrowserAnimationsModule,
      MatButtonModule,
    ],
    declarations: [AppComponent],
    providers: [],
    bootstrap: [AppComponent],
  })
  export class AppModule {}
  </code></pre>
  
  <p>app.component.ts:</p>
  
  <pre><code>import { Component } from '@angular/core';
  
  @Component({
    selector: 'app-root',
    templateUrl: 'app.component.html',
    styleUrls: ['app.component.scss'],
  })
  export class AppComponent {
    constructor() {}
  }
  </code></pre>
  
  <p>app.component.html</p>
  
  <pre><code>&lt;mat-button&gt;foo&lt;/mat-button&gt;
  </code></pre>
  
  <p>app.component.scss is empty.</p>
  
  <p>I know there are lots of posts about this. But nothing helped me, because I think I imported everything (<code>MatButtonModule</code>) correctly. Anyone an idea what I could do?</p>