NullReferenceException at AggregateBuilder() call for aggregates with private constructors

[a-shtifanov-laya]

Dear Community,

we found a strange behavior when AggregateBuilder() is null for single stream aggregates that do not have neither public constructor nor Create() method and when stream is aggregated in multiple parallel calls:

the aggregate:

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I forked your example repository and made some changes in Helpdesk sample.
a-shtifanov-laya/marten-concurrency-bug@39b8ca0

In order to reproduce this behavior:

• clone my forked repo https://github.com/a-shtifanov-laya/marten-concurrency-bug
• open it in IDE and navigate to Sample\Helpdesk\Helpdesk.Api project
• run from that directory
  • docker compose up -d postgres
  • dotnet run -- codegen write
• place a breakpoint in program.cs in a catch block (i.e. line 164)
  
• debug AggregateStreamInParallelOf test
  

you should land with exception in SingleStreamProjectionLiveAggregationXXXXXX like on my very first screenshot.

i'm wondering if it is a valid use case or we're doing something completely stupid that we get such an error (i.e. i had to disable optimistic check in order to reproduce it)

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I'm actually ready to contribute if you decide that this should be corrected and point me a bit into the places that i should look at..

Thanks!