fix: safely handle nested removal of menu routes (#19885)

MenuRegistry.filterClientViews removes inaccessible routes 
and their child routes. When iterating the list of all routes,
 a NPE can occur if a child route has already been 
removed together with its parent route. 
Add a check that prevents NPE for removed children.

flow-server/src/main/java/com/vaadin/flow/server/menu/MenuRegistry.java

@@ -340,6 +340,10 @@ private static void filterClientViews(
 
  Set<String> clientEntries = new HashSet<>(configurations.keySet());
  for (String key : clientEntries) {
+ if (!configurations.containsKey(key)) {
+ // view may have been removed together with parent
+ continue;
+ }
  AvailableViewInfo viewInfo = configurations.get(key);
  boolean routeValid = validateViewAccessible(viewInfo,
  isUserAuthenticated, vaadinRequest::isUserInRole);

flow-server/src/test/java/com/vaadin/flow/server/menu/MenuRegistryTest.java

@@ -131,6 +131,19 @@ public void getMenuItemsContainsExpectedClientPaths() throws IOException {
  assertClientRoutes(menuItems);
  }
 
+ @Test
+ public void getMenuItemsWithNestedFiltering_doesNotThrow()
+ throws IOException {
+ File generated = tmpDir.newFolder(GENERATED);
+ File clientFiles = new File(generated, FILE_ROUTES_JSON_NAME);
+ Files.writeString(clientFiles.toPath(), nestedLoginRequiredRouteFile);
+
+ Map<String, AvailableViewInfo> menuItems = new MenuRegistry()
+ .getMenuItems(true);
+
+ Assert.assertEquals(0, menuItems.size());
+ }
+
  @Test
  public void getMenuItemsNoFilteringContainsAllClientPaths()
  throws IOException {
@@ -462,4 +475,44 @@ public Instantiator getInstantiator() {
  }
  ]
  """;
+
+ String nestedLoginRequiredRouteFile = """
+ [
+ {
+ "route": "",
+ "params": {},
+ "title": "current Title",
+ "children": [
+ {
+ "route": "",
+ "loginRequired": true,
+ "params": {},
+ "title": "navigate"
+ },
+ {
+ "route": "admin",
+ "loginRequired": true,
+ "title": "Admin",
+ "params": {},
+ "children": [
+ {
+ "route": "planets",
+ "loginRequired": true,
+ "title": "Planets",
+ "params": {},
+ "children": [
+ {
+ "route": "",
+ "loginRequired": true,
+ "title": "Planets",
+ "params": {}
+ }
+ ]
+ }
+ ]
+ }
+ ]
+ }
+ ]
+ """;
 }