Address PR feedback

Author: noellie-velez

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

@@ -2226,7 +2226,8 @@ public bool TrySetParent(NetworkObject parent, bool worldPositionStays = true)
                 return false;
             }
 
-            if (!NetworkManager.IsListening)
+            var networkManager = NetworkManager;
+            if (!networkManager || !networkManager.IsListening)
             {
                 return false;
             }
@@ -2237,7 +2238,7 @@ public bool TrySetParent(NetworkObject parent, bool worldPositionStays = true)
 
             // If we don't have authority, and we are not shutting down, then don't allow any parenting.
             // If we are shutting down and don't have authority then allow it.
-            if (!isAuthority && !NetworkManager.ShutdownInProgress)
+            if (!isAuthority && !networkManager.ShutdownInProgress)
             {
                 return false;
             }