Fixes 189 #2, listbox should not be deleted also

fix #1 fixed label and error label, but getValue() was always returning
null!

Author: tbenbrahim2

src/main/java/gwt/material/design/addins/client/combobox/MaterialComboBox.java

@@ -217,7 +217,7 @@ public void clear() {
         final Iterator<Widget> it = iterator();
         while (it.hasNext()) {
         	final Widget widget = it.next();
-        	if (widget != label && widget != lblError) {
+        	if (widget != label && widget != lblError && widget != listbox) {
 	            it.remove();
         	}
         }