MatToolbar throws error when using it with Angular 9
Add below specific configuration in package.json and npm install.
{
"scripts": {
"postinstall": "ngcc"
}
}
Reference: https://angular.io/guide/ivy#speeding-up-ngcc-compilation
I had the same problem before, its because you modify your app.module.ts file while the server is running.
Try to stop it and then run it again using the ng servecommand.