Flink window evictor(TimeEvictor.of(Time.seconds(0))) 会出现 NullPoint问题

[HunterXHunter]

当程序使用
evictor(TimeEvictor.of(Time.seconds(0)))
来清除 窗口触发前数据时，当数据全部被清除了，在print时会报Null point

Caused by: java.lang.NullPointerException
        at
org.apache.flink.api.common.functions.util.PrintSinkOutputWriter.write(PrintSinkOutputWriter.java:73)
        at
org.apache.flink.streaming.api.functions.sink.PrintSinkFunction.invoke(PrintSinkFunction.java:81)


为什么没数据也会传一个 null 到 Sink？



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[HunterXHunter]

代码如下：
stream
.keyBy((KeySelector<Tuple2&lt;String, Long>, String>) o -> o.f0)
.window(TumblingEventTimeWindows.of(Time.seconds(100)))
.trigger(ContinuousProcessingTimeTrigger.of(Time.seconds(2)))
.evictor(TimeEvictor.of(Time.seconds(0)))
.sum(1)
.print();

当 数据在窗口计算前被全部清除时，sum结果会是一个null，会传入print，导致 nullpoint



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