Ignore windows that SwingUI doesnt know about when the client is starting up

Fixes issue 4203

Change-Id: I9bd2789ab3014af1de13f9d1e3e70f345da6cb33
Reviewed-on: http://gerrit.dmdirc.com/1296
Reviewed-by: Gregory Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>

src/com/dmdirc/addons/ui_swing/framemanager/buttonbar/ButtonBar.java

@@ -211,7 +211,9 @@ public final class ButtonBar implements FrameManager, ActionListener,
         for (FrameContainer<?> frame : windowCollection) {
             window = windowFactory.getSwingWindow(frame);
             parentWindow = windowFactory.getSwingWindow(frame.getParent());
-            windowAdded(parentWindow, window);
+            if (window != null) {
+                windowAdded(parentWindow, window);
+            }
 
             if (!frame.getChildren().isEmpty()) {
                 final ArrayList<FrameContainer<?>> childList = new ArrayList