Do detach() before clearing application (#9419)

server/src/com/vaadin/ui/UI.java

@@ -691,13 +691,14 @@ public abstract class UI extends AbstractComponentContainer implements
         if ((application == null) == (this.application == null)) {
             throw new IllegalStateException("Application has already been set");
         } else {
+            if (application == null) {
+                detach();
+            }
             this.application = application;
         }
 
         if (application != null) {
             attach();
-        } else {
-            detach();
         }
     }