Bugfix in prism.Result.

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@1832 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/prism/Result.java

@@ -129,6 +129,6 @@ public class Result
 	 */
 	public String toString()
 	{
-		return result.toString();
+		return result == null ? "null" : result.toString();
 	}
 }