title: No UIProvider has been added and there is no "UI" init parameter Vaadin error after Spring integration
body: <p>I recived No UIProvider has been added and there is no "UI" init parameter error after I tried dependency injection in Vaadin Framework. I used dedicated Vaadin Spring Addon. I also changed VaadinServlet to SpringVaadinServlet, still doesn't work.</p>
  
  <p>There is my MainView:</p>
  
  <pre><code>@Theme("mytheme")
  @SpringUI
  @ComponentScan
  @Widgetset("net.elenx.MyAppWidgetset")
  public class MainView extends UI {
  
      @Autowired
      private VerticalLayout template;
  
      @Override
      protected void init(VaadinRequest vaadinRequest) {
          new AnnotationConfigApplicationContext(SpringConfig.class);
          this.setContent(template);
      }
  
      @WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
      @VaadinServletConfiguration(ui = MainView.class, productionMode = false)
      public static class MyUIServlet extends SpringVaadinServlet {
      }
  
  }
  </code></pre>
  
  <p>NavigationBar</p>
  
  <pre><code>@Configuration
  @EnableVaadin
  public class NavigationBar {
  
      @Bean
      HorizontalLayout navigationBar(Button hamburgerButton, Label elenxLogo) {
          System.out.println("Hello from NavigationBar bean!");
          HorizontalLayout navbar = new HorizontalLayout();
          navbar.setWidth("100%");
          navbar.setMargin(true);
          navbar.setHeight(50, Sizeable.Unit.PIXELS);
          navbar.addComponent(hamburgerButton);
          navbar.addComponent(elenxLogo);
          navbar.addStyleName("navigation-bar");
          return navbar;
      }
  
      @Bean
      Button hamburgerButton() {
          Button hamburgerButton = new Button();
          hamburgerButton.addStyleName("hamburger-button");
          hamburgerButton.setIcon(VaadinIcons.MENU);
          return hamburgerButton;
      }
  
      @Bean
      Label elenxLogo() {
          Label logo = new Label("ElenX");
          logo.addStyleName("elenx-logo");
          logo.setWidthUndefined();
          logo.setEnabled(false);
          return logo;
      }
  }
  </code></pre>
  
  <p>SpringConfig</p>
  
  <pre><code>@Configuration
  @EnableVaadin
  @Import(NavigationBar.class)
  public class SpringConfig {
  
      //Create whole view of MainView
      @Bean
      VerticalLayout template(HorizontalLayout navigationBar) {
          System.out.println("Hello from template bean!");
          VerticalLayout template = new VerticalLayout();
          //NavigationBar navigationBar = new NavigationBar();
          Sidebar sidebar = new Sidebar();
          template.setMargin(false);
          template.setSpacing(false);
          template.setHeight("100%");
          template.addComponent(navigationBar);
          template.addComponent(sidebar.getSidebar());
          template.setExpandRatio(sidebar.getSidebar(), 1.0f);
          return template;
      }
  }
  </code></pre>
  
  <p>What is wrong with this code? I have no idea what's going on..</p>