fix NPE when enable unnecessary ReferenceDetection, for issue #3432

Author: yanxutao89

core/src/main/java/com/alibaba/fastjson2/JSONWriter.java

@@ -224,6 +224,9 @@ public final String setPath(int index, Object object) {
     }
 
     public final String setPath0(int index, Object object) {
+        if (path == null) {
+            return null;
+        }
         this.path = index == 0
                 ? (path.child0 != null ? path.child0 : (path.child0 = new Path(path, index)))
                 : index == 1

core/src/test/java/com/alibaba/fastjson2/issues_3400/Issue3432.java

@@ -0,0 +1,31 @@
+package com.alibaba.fastjson2.issues_3400;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.JSONWriter;
+import lombok.Data;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3432 {
+    @Test
+    public void test_toJSON() {
+        List<Node> nodes = new ArrayList<>();
+        Node node = new Node();
+        node.setGradeId("123");
+        node.setPrivilegeIds(Arrays.asList("1", "2"));
+        nodes.add(node);
+
+        assertEquals(com.alibaba.fastjson.JSON.toJSON(nodes), JSON.toJSON(nodes, JSONWriter.Feature.ReferenceDetection));
+    }
+
+    @Data
+    public static class Node {
+        private String gradeId;
+        private List<String> privilegeIds;
+    }
+}