Play framework 2.5.x: IllegalStateException: Attempted to call materialize() after the ActorMaterializer has been shut down

The intention of the Play server is to act as an intermediate service to another system, where the end-users access a small portion of functionality in a web browser on their smartphone.

The service functions as supposed to when connected to the real system. But, I want to have unit tests in a blackbox fashion. Testing the functionality from start to end. To avoid relying on the real system, where values change every day, I wanted to use the same approach as described in Testing web service clients and combine it with the route testing as shown in the example test from the play-java-seed, shown here for convenience:

public class HomeControllerTest extends WithApplication {

    @Override
    protected Application provideApplication() {
        return new GuiceApplicationBuilder().build();
    }

    @Test
    public void testIndex() {
        Http.RequestBuilder request = new Http.RequestBuilder()
                .method(GET)
                .uri("/");

        Result result = route(app, request);
        assertEquals(OK, result.status());
    }
}

My test code then looks as follows:

package controllers;

import java.util.*;

import org.junit.*;
import static org.junit.Assert.*;

import play.Application;
import play.inject.guice.GuiceApplicationBuilder;
import play.mvc.Result;
import play.mvc.Http.RequestBuilder;
import play.routing.*;
import play.server.Server;
import play.test.WithApplication;
import static play.mvc.Http.Status.*;
import static play.mvc.Results.*;
import static play.test.Helpers.*;

public class ApplicationControllerTest extends WithApplication {
  static int FAKE_SERVICE_PORT = 7070;
  Server server;

  @Override
  protected Application provideApplication() {
    return new GuiceApplicationBuilder()
      .configure("externalSystem.port", FAKE_SERVICE_PORT)
      .configure("externalSystem.host", "localhost")
      .build();
  }

  @Before
  public void setup() {
    Router router = new RoutingDsl()
      .GET("/b/1/").routeTo(() ->
        // Request for branch information
        ok().sendResource("externalSystem/getBranch.json")
      )
      .GET("/b/1/a/19033/").routeTo(() ->
        // Request generating an already arrived appointment
        ok().sendResource("externalSystem/arrivedAppointment.json")
      )
      .GET("/b/1/a/19962/").routeTo(() ->
        // Request generating a newly created appointment
        ok().sendResource("externalSystem/createdAppointment.json")
      )
      .build();
    server = Server.forRouter(router, FAKE_SERVICE_PORT);
  }

  @After
  public void tearDown() throws Exception {
    server.stop();
  }

  @Test
  public void AppCtrl() throws Exception {
    // // These values are defined in the json files
    String customerName = "Test User";
    String branchName = "Test Branch";

    RequestBuilder request = new RequestBuilder().method(GET).uri("/1/19962");

    Result result = route(app, request);
    assertEquals(OK, result.status());
    assertTrue(contentAsString(result).contains(customerName));
    assertTrue(contentAsString(result).contains(branchName));
  }
}

However running this test with sbt test produces an IllegalStateException:

[error] Test controllers.ApplicationControllerTest.AppCtrl failed: java.lang.IllegalStateException: Attempted to call materialize() after the ActorMaterializer has been shut down., took 4.165 sec
[error]     at akka.stream.impl.ActorMaterializerImpl.materialize(ActorMaterializerImpl.scala:164)
[error]     at akka.stream.impl.ActorMaterializerImpl.materialize(ActorMaterializerImpl.scala:146)
[error]     at akka.stream.scaladsl.RunnableGraph.run(Flow.scala:350)
[error]     at play.api.libs.streams.DoneAccumulator.run(Accumulator.scala:161)
[error]     at play.api.test.EssentialActionCaller$class.call(Helpers.scala:225)
[error]     at play.api.test.Helpers$.call(Helpers.scala:382)
[error]     at play.api.test.RouteInvokers$class.route(Helpers.scala:247)
[error]     at play.api.test.Helpers$.route(Helpers.scala:382)
[error]     at play.api.test.RouteInvokers$class.jRoute(Helpers.scala:234)
[error]     at play.api.test.Helpers$.jRoute(Helpers.scala:382)
[error]     at play.api.test.Helpers.jRoute(Helpers.scala)
[error]     at play.test.Helpers.route(Helpers.java:360)
[error]     at play.test.Helpers.route(Helpers.java:355)
[error]     at controllers.ApplicationControllerTest.AppCtrl(ApplicationControllerTest.java:63)
[error]     ...
[error] Failed: Total 1, Failed 1, Errors 0, Passed 0
[error] Failed tests:
[error]         controllers.ApplicationControllerTest
[error] (test:test) sbt.TestsFailedException: Tests unsuccessful

I guess I could implement a completely standalone server to mock the service, but I would much rather not have the tests rely on a separate server.

So my question is this: Has anyone done someting similar with Play, and in that case what have I done wrong?