fix: splitting the internal and visible parts of OnNetworkSpawn

com.unity.netcode.gameobjects/Runtime/Core/NetworkBehaviour.cs

@@ -436,6 +436,10 @@ internal void InternalOnNetworkSpawn()
             IsSpawned = true;
             InitializeVariables();
             UpdateNetworkProperties();
+        }
+
+        internal void VisibleOnNetworkSpawn()
+        {
             OnNetworkSpawn();
         }
 

com.unity.netcode.gameobjects/Runtime/Core/NetworkObject.cs

@@ -821,6 +821,13 @@ internal void InvokeBehaviourNetworkSpawn()
                     Debug.LogWarning($"{ChildNetworkBehaviours[i].gameObject.name} is disabled! Netcode for GameObjects does not support spawning disabled NetworkBehaviours! The {ChildNetworkBehaviours[i].GetType().Name} component was skipped during spawn!");
                 }
             }
+            for (int i = 0; i < ChildNetworkBehaviours.Count; i++)
+            {
+                if (ChildNetworkBehaviours[i].gameObject.activeInHierarchy)
+                {
+                    ChildNetworkBehaviours[i].VisibleOnNetworkSpawn();
+                }
+            }
         }
 
         internal void InvokeBehaviourNetworkDespawn()

com.unity.netcode.gameobjects/Runtime/NetworkVariable/NetworkVariableBase.cs

@@ -1,4 +1,5 @@
 using System;
+using UnityEngine;
 
 namespace Unity.Netcode
 {
@@ -79,7 +80,15 @@ protected NetworkVariableBase(
         public virtual void SetDirty(bool isDirty)
         {
             m_IsDirty = isDirty;
-            if (m_IsDirty && m_NetworkBehaviour != null)
+
+            if (m_NetworkBehaviour == null)
+            {
+                Debug.LogWarning($"NetworkVariable is written to, but doesn't know its NetworkBehaviour yet. " +
+                                 "Are you modifying a NetworkVariable before the NetworkObject is spawned?");
+                return;
+            }
+
+            if (m_IsDirty)
             {
                 m_NetworkBehaviour.NetworkManager.MarkNetworkObjectDirty(m_NetworkBehaviour.NetworkObject);
             }