explicit: Proper exception instead of null pointer exception for explicit with system definition

git-svn-id: https://www.prismmodelchecker.org/svn/prism/prism/trunk@11161 bbc10eb1-c90d-0410-af57-cb519fbb1720

prism/src/prism/Prism.java

@@ -1910,6 +1910,9 @@ public class Prism extends PrismComponent implements PrismSettingsListener
 					currentModel = mod2mtbdd.translate();
 					currentModelExpl = null;
 				} else {
+					if (currentModulesFile.getSystemDefn() != null) {
+						throw new PrismNotSupportedException("Explicit engine does not currently support the system...endsystem construct");
+					}
 					ConstructModel constructModel = new ConstructModel(this);
 					constructModel.setFixDeadlocks(getFixDeadlocks());
 					currentModelExpl = constructModel.constructModel(currentModelGenerator);