always set a background for docking area

dockinginfonode/src/main/java/fr/soleil/docking/infonode/InfoNodeDockingManager.java

 package fr.soleil.docking.infonode;
 
+import java.awt.Color;
 import java.beans.PropertyChangeEvent;
 import java.io.ByteArrayInputStream;
 import java.io.ByteArrayOutputStream;
@@ -40,14 +41,17 @@ public class InfoNodeDockingManager extends ADockingManager {
     @Override
     public JComponent getDockingArea() {
         if (rootWindow == null) {
-            rootWindow = (RootWindow) createNewDockingArea();
+            rootWindow = (RootWindow) createNewDockingArea(null);
         }
         return rootWindow;
     }
 
     @Override
-    public JComponent createNewDockingArea() {
+    public JComponent createNewDockingArea(Color background) {
         RootWindow result = new RootWindow(new MyViewSerializer(viewFactory));
+        if (background != null) {
+            result.getRootWindowProperties().getWindowAreaProperties().setBackgroundColor(background);
+        }
         for (IView view : viewFactory.getViews()) {
             DockingUtil.addWindow((View) view, rootWindow);
         }