one more null check

Author: rbri

src/main/java/org/htmlunit/javascript/PostponedAction.java

@@ -61,7 +61,9 @@ protected Page getOwningPage() {
      */
     public boolean isStillAlive() {
         final Page owningPage = getOwningPage();
-        return owningPage != null && owningPage == owningPage.getEnclosingWindow().getEnclosedPage();
+        return owningPage != null
+                && owningPage.getEnclosingWindow() != null
+                && owningPage == owningPage.getEnclosingWindow().getEnclosedPage();
     }
 
     @Override