NullReferenceException while building a project #1378

Describe the bug
Getting the following exception (in Editor.log) when building one of our projects after only importing the Netcode package. A minimal, new Unity project with just the package imported works fine though.

(0,0): error System.NullReferenceException: Object reference not set to an instance of an object
at Unity.Netcode.Editor.CodeGen.NetworkBehaviourILPP.ImportReferences (Mono.Cecil.ModuleDefinition moduleDefinition) [0x0069e] in <1d1144dabdc047fba7e8e2c6b3bfbaa5>:0
at Unity.Netcode.Editor.CodeGen.NetworkBehaviourILPP.Process (Unity.CompilationPipeline.Common.ILPostProcessing.ICompiledAssembly compiledAssembly) [0x000a1] in <1d1144dabdc047fba7e8e2c6b3bfbaa5>:0
at ILPostProcessorRunner.RunILPostProcessors (NamedILPostProcessorWrapper[] ilpostProcessors) [0x00073] in <04671920fa244b7da7c644c861e4f88f>:0
at ILPostProcessorRunner.Main (System.String[] args) [0x000be] in <04671920fa244b7da7c644c861e4f88f>:0

Screenshots

Environment (please complete the following information):

• OS: Windows 10
• Unity Version: 2020.3.13f1
• Netcode Version: 1.0.0-pre2
• Netcode Commit: ?

I've already tried investigating the issue but I couldn't get the Debugger to enter the ImportReferences() method. And there are many places in that method where we could get an NRE 😉

Any help would be greatly appreciated.