Fix errors

Content.Shared/Pulling/Components/PullableComponent.cs

@@ -24,7 +24,7 @@ namespace Content.Shared.Pulling.Components
         /// The current entity pulling this component.
         /// SharedPullingStateManagementSystem should be writing this. This means definitely not you.
         /// </summary>
-        public EntityUid Puller { get; set; }
+        public EntityUid? Puller { get; set; }
         /// <summary>
         /// The pull joint.
         /// SharedPullingStateManagementSystem should be writing this. This means probably not you.

Content.Shared/Pulling/Components/SharedPullerComponent.cs

@@ -17,7 +17,7 @@ namespace Content.Shared.Pulling.Components
         public float SprintSpeedModifier => Pulling == default ? 1.0f : 0.75f;
 
         [ViewVariables]
-        public EntityUid Pulling { get; set; }
+        public EntityUid? Pulling { get; set; }
 
         protected override void Shutdown()
         {

Content.Shared/Pulling/Systems/SharedPullableSystem.cs

@@ -20,7 +20,7 @@ namespace Content.Shared.Pulling.Systems
         private void OnRelayMoveInput(EntityUid uid, SharedPullableComponent component, RelayMoveInputEvent args)
         {
             var entity = args.Session.AttachedEntity;
-            if (!entity.IsValid() || !_blocker.CanMove(entity)) return;
+            if (!entity.HasValue || !_blocker.CanMove(entity.Value)) return;
             _pullSystem.TryStopPull(component);
         }
     }