bugfix return

src/main/java/astGenerator/ASTGenerator.java

@@ -140,8 +140,12 @@ public class ASTGenerator extends DecafBaseVisitor<Node> {
 
     @Override
     public Node visitReturnStmt(DecafParser.ReturnStmtContext ctx) {
+        if (ctx.expression() != null) {
             Node expression = visitExpression(ctx.expression());
             return new ReturnStatement((IExpression) expression);
+        } else {
+            return new ReturnStatement(null);
+        }
     }
 
     @Override