Trap potential GUI bug (from Marco).

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@2394 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/simulator/SimulatorEngine.java

@@ -571,6 +571,10 @@ public class SimulatorEngine
 	 */
 	public Object queryProperty(int index)
 	{
+		if (index < 0 || index >= propertySamplers.size()) {
+			System.out.println("Warning: Can't query property " + index);
+			return null;
+		}
 		Sampler sampler = propertySamplers.get(index);
 		return sampler.isCurrentValueKnown() ? sampler.getCurrentValue() : null;
 	}