fix #4071

src/main/java/com/alibaba/druid/sql/visitor/SQLASTOutputVisitor.java

@@ -2939,7 +2939,9 @@ public class SQLASTOutputVisitor extends SQLASTVisitorAdapter implements Paramet
         }
 
         SQLSelect select = stmt.getSelect();
-        this.visit(select);
+        if (select != null) {
+            this.visit(select);
+        }
 
         return false;
     }

src/test/java/com/alibaba/druid/bvt/bug/Issue4071.java

@@ -0,0 +1,10 @@
+package com.alibaba.druid.bvt.bug;
+
+import com.alibaba.druid.sql.ast.statement.SQLSelectStatement;
+import junit.framework.TestCase;
+
+public class Issue4071 extends TestCase {
+    public void test_for_issue() throws Exception {
+        assertEquals("",new SQLSelectStatement().toString());
+    }
+}