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 ...

We show you how to fix Object reference not set to an instance of an object error prompt which you may see in Microsoft Visual Studio.

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

Alexandre Tolskenko implemented successfully for unity and he wrote that the code had to be rewritten without any reference to windows, to drop everything related to networking, threading, imaging etc ...

Object Reference Not Set to an Instance of an Object on Windows Interestingly, this issue isn’t exclusive to Microsoft Visual Studio and may appear in other applications relying on Visual Studio ...