if counter-example is set to NULL because of an exception on the Prolog side,...

if counter-example is set to NULL because of an exception on the Prolog side, the DisproverReasoner no longer throws a NullPointerException but fails with an appropriate error message

de.prob.eventb.disprover.core/src/de/prob/eventb/disprover/core/DisproverReasoner.java

@@ -134,6 +134,11 @@ public class DisproverReasoner implements IReasoner {
 
 		IAntecedent ante = ProverFactory.makeAntecedent(goal);
 		
+		if (counterExample == null) {
+			return ProverFactory.reasonerFailure(this, input,
+					"ProB: Error occurred.");
+		}
+
 		if (counterExample.timeoutOccured()) {
 			System.out.println(sequent.toString() + ": Timeout occured.");
 			return ProverFactory.reasonerFailure(this, input,