pnowojski commented on PR #20158:
URL: https://github.com/apache/flink/pull/20158#issuecomment-1176038063

   This code kind of always assumed serializers can be null. Both input and 
output. Originally, before we introduced multiple input operators, it kind of 
held together, as the `StreamNode` had three fields: `typeSerializerIn1`, 
`typeSerializerIn2` and `typeSerializerOut`. Either of those could be null. 
When introducing multiple inputs, I squashed the `typeSerializerInX` fields 
into an array, unfortunately also preserving nulls. 
   
   I've looked into disallowing nulls into those setters, but that made the 
caller code quite ugly, with lots of ifs. And those nulls are used in a quite a 
lot of places (check `StreamGraph#setSerializers(...)` calls)


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