Redirect from external website and user interface instance

Flow
1

Hello!
Please describe to me the best solution for the following situation.

  1. The user creates an order on my site
  2. The user goes to the payment gateway website (app?.page?.setLocation(url)) and makes the payment
  3. The payment gateway makes a redirect with the parameters back to my site, the parameters are processed and the user returns to the main page, preferably with a preserved interface.

I have a problem with point three. The request from the payment gateway comes to:

@Route("payment")
class PaymentView(...): Div(), RouterLayout, HasUrlParameter<String>

parameters are processed successfully, the user is shown a dialog with payment information and “Return”, but when he clicks the return button to the main window of the site,

fun return() {
        close() //close dialog
        UI.getCurrent().access { // or without UI.getCurrent().access
            UI.getCurrent().page.executeJs("window.location.href = ''") 
            // or UI.getCurrent().page.setLocation("https://site.com/main") 
	        // or UI.getCurrent().navigate(MainView::class.java)
        }
    }

UI is not displayed - just blank screen, but the address line changes correctly. When you press the f5 key to update your browser, an exception occurs:

ERROR InternalServerError There was an exception while trying to navigate to 'main'
java.lang.NullPointerException: null
  at com.github.appreciated.app.layout.router.AppLayoutRouterLayoutBase.showRouterLayoutContent(AppLayoutRouterLayoutBase.java:51)
  at com.vaadin.flow.component.internal.UIInternals.showRouteTarget(UIInternals.java:711)
  at com.vaadin.flow.router.internal.AbstractNavigationStateRenderer.handle(AbstractNavigationStateRenderer.java:252)
  at com.vaadin.flow.router.Router.handleNavigation(Router.java:249)
  at com.vaadin.flow.router.Router.navigate(Router.java:220)
  at com.vaadin.flow.router.Router.navigate(Router.java:186)
  at com.vaadin.flow.router.Router.initializeUI(Router.java:93)
  at com.vaadin.flow.server.BootstrapHandler.createAndInitUI(BootstrapHandler.java:1516)
  at com.vaadin.flow.server.BootstrapHandler.synchronizedHandleRequest(BootstrapHandler.java:472)
  at com.vaadin.flow.server.SynchronizedRequestHandler.handleRequest(SynchronizedRequestHandler.java:40)
  at com.vaadin.flow.server.VaadinService.handleRequest(VaadinService.java:1545)
  at com.vaadin.flow.server.VaadinServlet.service(VaadinServlet.java:247)
  at com.vaadin.flow.spring.SpringServlet.service(SpringServlet.java:120)
  at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)

After returning from an external site, is the user in another UI instance? How to switch to the previous one UI, where the whole site interface is?
I tried to save and load the ui from session - without results:

VaadinSession.getCurrent().setAttribute("current_ui", ui) //before redirect to external website
	...
	val ui = VaadinSession.getCurrent().getAttribute("current_ui") as UI?	
        UI.setCurrent(ui)

In the previous version of the site on Vaadin 6, requests were processed as follows:

public void transactionStart(Application application, Object transactionData) {
            getPaymentRequestParameters((HttpServletRequest) transactionData); //parameters processing, ui updating 
    }

I found a similar situation on the forum, but without an answer
[https://vaadin.com/forum/thread/10779604/navigate-to-view-from-requesthandler-or-servlet]
(https://vaadin.com/forum/thread/10779604/navigate-to-view-from-requesthandler-or-servlet) .

My project uses Vaadin 14.2.1 in compatibilityMode=true and productionMode=true, Spring Boot 2.2.1 with Tomcat. MainView is annotated as
@PreserveOnRefresh, @SpringComponent, @UIScope