fastson 1.2.73 NullPointerException when set AfterFilter

[leonchen83]

it works in 1.2.72
reproduce code

import com.alibaba.fastjson.serializer.AfterFilter;
import com.alibaba.fastjson.serializer.JSONSerializer;
import com.alibaba.fastjson.serializer.SerializeConfig;
import com.alibaba.fastjson.serializer.SerializeWriter;
import com.alibaba.fastjson.serializer.SerializerFeature;

public class Main {
	public static void main(String[] args) {
		Json json = new Json();
		json.setOp(new Operator());
		try(SerializeWriter out = new SerializeWriter()) {
			JSONSerializer s = new JSONSerializer(out, new SerializeConfig());
			s.config(SerializerFeature.DisableCircularReferenceDetect, true);
			s.getAfterFilters().add(new JsonFilter());
			s.write(json);
			System.out.println(out.toString());
		}
	}
	
	static class Json {
		private Operator op;

		public Operator getOp() {
			return op;
		}

		public void setOp(Operator op) {
			this.op = op;
		}
	}
	
	static class Operator {
	}
	
	static class JsonFilter extends AfterFilter {
		
		@Override
		public void writeAfter(Object object) {
			if(object instanceof Operator) {
				writeKeyValue("operator", "operator");
			}
		}
	}
}

exception stack

Exception in thread "main" com.alibaba.fastjson.JSONException: write javaBean error, fastjson version 1.2.73, class cn.nextop.thorin.admin.json.Main$Json, write javaBean error, fastjson version 1.2.73, class cn.nextop.thorin.admin.json.Main$Operator, fieldName : op
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:539)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:149)
	at com.alibaba.fastjson.serializer.JSONSerializer.write(JSONSerializer.java:312)
	at cn.nextop.thorin.admin.json.Main.main(Main.java:41)
Caused by: com.alibaba.fastjson.JSONException: write javaBean error, fastjson version 1.2.73, class cn.nextop.thorin.admin.json.Main$Operator, fieldName : op
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:539)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:149)
	at com.alibaba.fastjson.serializer.FieldSerializer.writeValue(FieldSerializer.java:320)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:470)
	... 3 more
Caused by: java.lang.NullPointerException
	at com.alibaba.fastjson.serializer.AfterFilter.writeKeyValue(AfterFilter.java:26)
	at cn.nextop.thorin.admin.json.Main$JsonFilter.writeAfter(Main.java:70)
	at com.alibaba.fastjson.serializer.AfterFilter.writeAfter(AfterFilter.java:17)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.writeAfter(JavaBeanSerializer.java:834)
	at com.alibaba.fastjson.serializer.JavaBeanSerializer.write(JavaBeanSerializer.java:502)
	... 6 more

fastjson version: 1.2.73
jdk version: openjdk-11.0.2