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 ...

OctopusCreateRelease@6 - Failed to execute command. Object reference not set to an instance of an object. #328 ...

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.