fix

NetworkAnimator started throwing this exception:
"Objects are trying to be loaded during a domain backup. This is not allowed as it will lead to undefined behaviour!"
When MPPM was being used. This has to do with the OnBeforeSerialize method and potentially an order of operations issue.
We can accomplish the same serialization within OnValidate, but went ahead and made it virtual in order to provide users with the ability to override and handle their own validation.

Author: NoelStephensUnity

com.unity.netcode.gameobjects/Runtime/Components/NetworkAnimator.cs

@@ -298,14 +298,11 @@ private void ParseStateMachineStates(int layerIndex, ref AnimatorController anim
             }
         }
 
-#endif
-
         /// <summary>
         /// Creates the TransitionStateInfoList table
         /// </summary>
         private void BuildTransitionStateInfoList()
         {
-#if UNITY_EDITOR
             if (m_Animator == null)
             {
                 return;
@@ -323,17 +320,26 @@ private void BuildTransitionStateInfoList()
                 var stateMachine = animatorController.layers[x].stateMachine;
                 ParseStateMachineStates(x, ref animatorController, ref stateMachine);
             }
-#endif
         }
 
+        /// <summary>
+        /// In-Editor Only
+        /// Virtual OnValidate method for custom derived NetworkAnimator classes.
+        /// </summary>
+        protected virtual void OnValidate()
+        {
+            BuildTransitionStateInfoList();
+        }
+#endif
+
         public void OnAfterDeserialize()
         {
             BuildDestinationToTransitionInfoTable();
         }
 
         public void OnBeforeSerialize()
         {
-            BuildTransitionStateInfoList();
+            // Do nothing when serializing (handled during OnValidate)
         }
 
         internal struct AnimationState : INetworkSerializable