Don't try and remove windows that don't exist in swing UI

Change-Id: Idac4020e7fa766f28f829f97310f2b0f9e7ba37f
Reviewed-on: http://gerrit.dmdirc.com/1775
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

@@ -208,7 +208,8 @@ public final class WindowMenuFrameManager extends JMenu implements
                 item = menus.get(window.getContainer());
                 menus.remove(window.getContainer());
             } else {
-                item = null;
+                window.getContainer().removeSelectionListener(this);
+                return;
             }
             UIUtilities.invokeLater(new Runnable() {