Check window exists before using it

Fixes issue 4457

Change-Id: Iae1297c043b2c65836db2c276cb39d71cce827aa
Reviewed-on: http://gerrit.dmdirc.com/1504
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Chris Smith <chris@dmdirc.com>

src/com/dmdirc/addons/ui_swing/framemanager/windowmenu/WindowMenuFrameManager.java

@@ -357,7 +357,10 @@ public final class WindowMenuFrameManager extends JMenu implements
      * @param window parent to inform
      */
     protected void parentSelection(final FrameContainer window) {
-        menus.get(window).childSelected();
+        final FrameContainerMenu menuItem = menus.get(window);
+        if (menuItem != null) {
+            menuItem.childSelected();
+        }
     }
 
     /**