I'm also encountering this issue. It specifically occurs when Unity tries to serialize a NetworkVariable<FixedString32Bytes>. I am able to set the value of the network variable and read it again ...

I have Unity 2017.2.0b5 (64-bit) and the latest Github for Unity release. Any additional information, configuration or data that might be necessary to reproduce the issue.

Ошибка в Unity NullReferenceException: Object reference not set to an instance of an object ...