'Found the synthetic property @panelState. Please include either "BrowserAnimationsModule" or "NoopAnimationsModule" in your application.'

I upgraded an Angular 4 project using angular-seed and now get the error

Found the synthetic property @panelState. Please include either "BrowserAnimationsModule" or "NoopAnimationsModule" in your application.

How can I fix this? What exactly is the error message telling me?

Make sure the @angular/animations package is installed (e.g. by running npm install @angular/animations). Then, in your app.module.ts

import { BrowserAnimationsModule } from '@angular/platform-browser/animations';

@NgModule({
  ...,
  imports: [
    ...,
    BrowserAnimationsModule
  ],
  ...
})