Assert that connector needing layout is attached (#11698)

Change-Id: Ie2d1ec330b857497c7673f3805e35b452f409f3c

client/src/com/vaadin/client/LayoutManager.java

@@ -827,6 +827,7 @@ public class LayoutManager {
      *            the managed layout that should be layouted
      */
     public final void setNeedsHorizontalLayout(ManagedLayout layout) {
+        assert isAttached(layout);
         needsHorizontalLayout.add(layout.getConnectorId());
     }
 
@@ -842,9 +843,21 @@ public class LayoutManager {
      *            the managed layout that should be layouted
      */
     public final void setNeedsVerticalLayout(ManagedLayout layout) {
+        assert isAttached(layout);
         needsVerticalLayout.add(layout.getConnectorId());
     }
 
+    private boolean isAttached(ServerConnector connector) {
+        while (connector != null) {
+            connector = connector.getParent();
+            if (connector == connection.getUIConnector()) {
+                return true;
+            }
+        }
+        // Reaching null parent before reaching UI connector -> not attached
+        return false;
+    }
+
     /**
      * Gets the outer height (including margins, paddings and borders) of the
      * given element, provided that it has been measured. These elements are