Catch possible null value in GUI simulator update table display (reported by Steffen Marcker).

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@10833 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/userinterface/simulator/GUISimulatorUpdatesTable.java

@@ -163,7 +163,7 @@ public class GUISimulatorUpdatesTable extends JTable implements ListSelectionLis
 
 		public Component getTableCellRendererComponent(JTable table, Object value, boolean isSelected, boolean hasFocus, int row, int column)
 		{
-			renderer.setText(value.toString());
+			renderer.setText(value == null ? "" : value.toString());
 
 			int dist;