hotfix: 统计出现NPE

msg-center-api/src/main/java/cn/axzo/msg/center/service/pending/response/PendingMessageStatisticResponseV2.java

@@ -9,6 +9,7 @@ import lombok.Setter;
 
 import java.io.Serializable;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -34,7 +35,7 @@ public class PendingMessageStatisticResponseV2 implements Serializable {
     /**
      * 分类情况
      */
-    private List<GroupStat> groupStats = new ArrayList<>();
+    private List<GroupStat> groupStats = Collections.synchronizedList(new ArrayList<>());
 
     public void addGroupStat(GroupStat groupStat) {
         this.groupStats.add(groupStat);