Default value of new properties file label in GUI is now "true" so adding a new label is less problematic.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@162 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/userinterface/properties/GUIPropLabelList.java

@@ -176,7 +176,7 @@ public class GUIPropLabelList extends JTable
 		
 		public void addLabel()
 		{
-			labels.add(new GUILabel("L"+labCount, null));
+			labels.add(new GUILabel("L"+labCount, new ExpressionTrue()));
 			parent.setModified(true);
 			fireTableRowsInserted(labels.size()-1, labels.size()-1);
 			labCount++;