Fixed twice registered UIProvider


## Example
* Disable mapping in VaadinConfig.groovy

    //    mapping = [
    //            "/*": "app.MyUI"
    //    ]

* Create the following three Groovy classes

### DemoUI.groovy
    @VaadinUI(path = "/demo")
    class DemoUI extends UI {
    
        @Override
        protected void init(VaadinRequest vaadinRequest) { }
    }

### DemoView.groovy
    @VaadinView(path = "demo", ui = DemoUI)
    class DemoView extends CustomComponent implements View {
    
        @Override
        void enter(ViewChangeListener.ViewChangeEvent viewChangeEvent) {
            def layout = new VerticalLayout()
            layout.setMargin(true)
            layout.setSpacing(true)
            layout.addComponent(new Label(value: "View Nr. 1", styleName: "h1"))
            layout.addComponent(new Button("Go forward", new Button.ClickListener() {
                @Override
                void buttonClick(Button.ClickEvent clickEvent) {
                    Views.enter(Demo2View)
                }
            }))
            compositionRoot = layout
            Page.current.setTitle("Demo View Nr. 1")
        }
    }

### Demo2View.groovy
    @VaadinView(path = "demo2", ui = DemoUI)
    class Demo2View extends CustomComponent implements View {
    
        @Override
        void enter(ViewChangeListener.ViewChangeEvent viewChangeEvent) {
            def layout = new VerticalLayout()
            layout.setMargin(true)
            layout.setSpacing(true)
            layout.addComponent(new Label(value: "View Nr. 2", styleName: "h1"))
            def paramField = new TextField("Some parameter")
            layout.addComponent(paramField)
            layout.addComponent(new Button("Go back", new Button.ClickListener() {
                @Override
                void buttonClick(Button.ClickEvent clickEvent) {
                    Views.enter(DemoView, [someparam: paramField.value])
                }
            }))
            compositionRoot = layout
            Page.current.setTitle("Demo View Nr. 2")
        }
    }