HandComponents now have a default hand, fixes crash

Content.Client/GameObjects/Components/Items/ClientHandsComponent.cs

@@ -59,7 +59,7 @@ namespace Content.Client.GameObjects
 
         public IEntity GetEntity(string index)
         {
-            if (_hands.TryGetValue(index, out var entity))
+            if (!string.IsNullOrEmpty(index) && _hands.TryGetValue(index, out var entity))
             {
                 return entity;
             }
@@ -166,6 +166,8 @@ namespace Content.Client.GameObjects
             {
                 _hands.Add(slot, null);
             }
+
+            serializer.DataField(this, x => ActiveIndex, "defaultHand", _hands.Keys.LastOrDefault());
         }
 
         public override void HandleMessage(ComponentMessage message, IComponent component)

Content.Server/GameObjects/Components/GUI/ServerHandsComponent.cs

@@ -3,6 +3,7 @@
 using Robust.Shared.Utility;
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Content.Server.GameObjects.EntitySystems;
 using Content.Server.Interfaces.GameObjects;
 using Content.Shared.GameObjects;
@@ -68,6 +69,8 @@ namespace Content.Server.GameObjects
                     AddHand(handsname);
                 }
             }
+
+            serializer.DataField(ref activeIndex, "defaultHand", orderedHands.LastOrDefault());
         }
 
         public IEnumerable<ItemComponent> GetAllHeldItems()