fixed apcnet causing crash (#3354)

Co-authored-by: Pieter-Jan Briers <pieterjan.briers+git@gmail.com>

Content.Server/GameObjects/EntitySystems/ApcNetSystem.cs

@@ -4,12 +4,13 @@ using JetBrains.Annotations;
 using Robust.Shared.GameObjects;
 using Robust.Shared.IoC;
 using System.Collections.Generic;
+using Content.Shared.GameTicking;
 using Robust.Shared.Timing;
 
 namespace Content.Server.GameObjects.EntitySystems
 {
     [UsedImplicitly]
-    internal sealed class ApcNetSystem : EntitySystem
+    internal sealed class ApcNetSystem : EntitySystem, IResettingEntitySystem
     {
         [Dependency] private readonly IPauseManager _pauseManager = default!;
 
@@ -34,5 +35,12 @@ namespace Content.Server.GameObjects.EntitySystems
         {
             _apcNets.Remove(apcNet);
         }
+
+        public void Reset()
+        {
+            // NodeGroupSystem does not remake ApcNets affected during restarting until a frame later,
+            // when their grid is invalid. So, we are clearing them on round restart.
+            _apcNets.Clear();
+        }
     }
 }