instanceof check before casting to IInvariant

for getting descriptions

Signed-off-by: Michael Leuschel <leuschel@uni-duesseldorf.de>

de.prob.core/src/de/prob/eventb/translator/internal/ModelTranslator.java

@@ -580,6 +580,7 @@ public class ModelTranslator extends AbstractComponentTranslator {
 			if (isDefinedHere(evPredicate)) {
 				final PPredicate predicate = translatePredicate(ff, te,
 						evPredicate);
+				if (evPredicate.getSource() instanceof IInvariant) {  // test can fail for generated invariants
                     final IInvariant ucp = (IInvariant) evPredicate.getSource(); // comments only attached in unchecked source
                     if (ucp.hasAttribute(EventBAttributes.COMMENT_ATTRIBUTE)) {
                         final String commentString = ucp.getAttributeValue(EventBAttributes.COMMENT_ATTRIBUTE);
@@ -592,6 +593,10 @@ public class ModelTranslator extends AbstractComponentTranslator {
                     } else {
                         list.add(predicate);
                         labelMapping.put(predicate, evPredicate);
+                    }
+				} else { // cannot cast source to IInvariant
+					list.add(predicate);
+					labelMapping.put(predicate, evPredicate);
 				}
 			}
 		}