1 files changed, 10 insertions, 4 deletions
diff --git a/src/com/itmill/toolkit/ui/Window.java b/src/com/itmill/toolkit/ui/Window.java
index 8615480553..2d9ce6e9cf 100644
--- a/src/com/itmill/toolkit/ui/Window.java
+++ b/src/com/itmill/toolkit/ui/Window.java
@@ -1009,13 +1009,19 @@ public class Window extends Panel implements URIHandler, ParameterHandler {
     public void addWindow(Window window) throws IllegalArgumentException,
             NullPointerException {
 
-        if (getParent() != null) {
+        if (window == null) {
+            throw new NullPointerException("Argument must not be null");
+        }
+
+        if (application.getWindows().contains(window)) {
             throw new IllegalArgumentException(
-                    "You can only add windows inside application-level windows");
+                    "Window was already added to application"
+                            + " - it can not be added to another window also.");
         }
 
-        if (window == null) {
-            throw new NullPointerException("Argument must not be null");
+        if (getParent() != null) {
+            throw new IllegalArgumentException(
+                    "You can only add windows inside application-level windows");
         }
 
         subwindows.add(window);