fix: compile error with generic NetworkBehaviour singletons (#2603)

* fix: compile error with generic NetworkBehaviour singletons

* changelog

* Added another fix for a similar issue.

---------

Co-authored-by: Noel Stephens <noel.stephens@unity3d.com>

Author: ShadauxCat

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -16,6 +16,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 - Fixed issue where `NetworkObject.SpawnWithObservers` was not being honored for late joining clients. (#2623)
 - Fixed issue where invoking `NetworkManager.Shutdown` multiple times, depending upon the timing, could cause an exception. (#2622)
 - Fixed issue where removing ownership would not notify the server that it gained ownership. This also resolves the issue where an owner authoritative NetworkTransform would not properly initialize upon removing ownership from a remote client. (#2618)
+- Fixed an ILPP compile error when creating a generic NetworkBehaviour singleton with a static T instance. (#2603)
 
 ### Changed
 

com.unity.netcode.gameobjects/Editor/CodeGen/CodeGenHelpers.cs

@@ -59,7 +59,7 @@ public static uint Hash(this MethodDefinition methodDefinition)
 
         public static bool IsSubclassOf(this TypeDefinition typeDefinition, string classTypeFullName)
         {
-            if (!typeDefinition.IsClass)
+            if (typeDefinition == null || !typeDefinition.IsClass)
             {
                 return false;
             }
@@ -154,6 +154,10 @@ public static MethodReference MakeGeneric(this MethodReference self, params Type
 
         public static bool IsSubclassOf(this TypeReference typeReference, TypeReference baseClass)
         {
+            if (typeReference == null)
+            {
+                return false;
+            }
             var type = typeReference.Resolve();
             if (type?.BaseType == null || type.BaseType.Name == nameof(Object))
             {

com.unity.netcode.gameobjects/Editor/CodeGen/NetworkBehaviourILPP.cs

@@ -2225,6 +2225,12 @@ private void GenerateVariableInitialization(TypeDefinition type)
                     }
                     field = new FieldReference(fieldDefinition.Name, fieldDefinition.FieldType, genericType);
                 }
+
+                if (field.FieldType.Resolve() == null)
+                {
+                    continue;
+                }
+
                 if (!field.FieldType.IsArray && !field.FieldType.Resolve().IsArray && field.FieldType.IsSubclassOf(m_NetworkVariableBase_TypeRef))
                 {
                     // if({variable} == null) {