Steps to reproduce:

1. Open attached project "772633.zip"
2. Open scene "Assets/synclist/synclist.unity"
3. Run scene
4. Start as host
5. Notice errors in console:
NullReferenceException: Object reference not set to an instance of an object
PrefabCode.Awake ()
UnityEngine.Networking.NetworkIdentity:UNetStaticUpdate()

NullReferenceException: Object reference not set to an instance of an object
Unity.GeneratedNetworkCode._WriteStructASyncList_None (UnityEngine.Networking.NetworkWriter writer, .ASyncList value)
PrefabCode.OnSerialize (UnityEngine.Networking.NetworkWriter writer, Boolean forceAll)

In PrefabCode.cs change "public ASyncList list" to "public ASyncList list = new ASyncList();"

Reproduced with: 5.1.4f1, 5.2.4f1, 5.3.2p4, 5.4.0b7