fix return JSONObject instead of null, for issue #3419

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

@@ -210,7 +210,7 @@ public JSONObject getJSONObject(int index) {
             return writerAdapter.toJSONObject(value);
         }
 
-        return null;
+        return (JSONObject) JSON.toJSON(value);
     }
 
     /**

core/src/test/java/com/alibaba/fastjson2/issues_3400/Issue3419.java

@@ -0,0 +1,30 @@
+package com.alibaba.fastjson2.issues_3400;
+
+import com.alibaba.fastjson2.JSONArray;
+import com.alibaba.fastjson2.JSONObject;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue3419 {
+    @Test
+    public void test() {
+        Map<String, Object> map = new HashMap<>();
+        map.put("str", new float[] {1.0f, 1.0f, 1.0f, 1.0f, 1.0f});
+
+        com.alibaba.fastjson.JSONObject jo1 = new com.alibaba.fastjson.JSONObject();
+        com.alibaba.fastjson.JSONArray ja1 = new com.alibaba.fastjson.JSONArray();
+        ja1.addAll(map.entrySet());
+        jo1.put("content", ja1);
+
+        JSONObject jo2 = new JSONObject();
+        JSONArray ja2 = new JSONArray();
+        ja2.addAll(map.entrySet());
+        jo2.put("content", ja2);
+
+        assertEquals(jo1.getJSONArray("content").getJSONObject(0), jo2.getJSONArray("content").getJSONObject(0));
+    }
+}