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.

Object reference not set to an instance of an object. Copy errors encountered, copying will continue: 1 error(s) reported. Copied: PSD001:\Selection Profiles with Replace option Copied standard ...