fix: NetworkIdentity.RebuildObservers: added missing null check for o…

…bservers coming from components that implement OnRebuildObservers. Previously this caused a NullReferenceException.

Assets/Mirror/Runtime/NetworkIdentity.cs

@@ -1110,7 +1110,7 @@ public void RebuildObservers(bool initialize)
                 observers.Clear();
                 foreach (NetworkConnection conn in newObservers)
                 {
-                    if (conn.isReady)
+                    if (conn != null && conn.isReady)
                         observers.Add(conn.connectionId, conn);
                 }
             }