Fix followers getting sent to nullspace when target is polymorphed

Content.Server/Polymorph/Systems/PolymorphSystem.cs

@@ -267,6 +267,10 @@ private void OnDestruction(Entity<PolymorphedEntityComponent> ent, ref Destructi
         if (PausedMap != null)
             _transform.SetParent(uid, targetTransformComp, PausedMap.Value);
 
+        // Raise an event to inform anything that wants to know about the entity swap
+        var ev = new PolymorphedEvent(uid, child, false);
+        RaiseLocalEvent(uid, ref ev);
+
         return child;
     }
 
@@ -339,6 +343,10 @@ private void OnDestruction(Entity<PolymorphedEntityComponent> ent, ref Destructi
         // if an item polymorph was picked up, put it back down after reverting
         _transform.AttachToGridOrMap(parent, parentXform);
 
+        // Raise an event to inform anything that wants to know about the entity swap
+        var ev = new PolymorphedEvent(uid, parent, true);
+        RaiseLocalEvent(uid, ref ev);
+
         _popup.PopupEntity(Loc.GetString("polymorph-revert-popup-generic",
                 ("parent", Identity.Entity(uid, EntityManager)),
                 ("child", Identity.Entity(parent, EntityManager))),

Content.Shared/Follower/FollowerSystem.cs

@@ -6,6 +6,7 @@
 using Content.Shared.Hands;
 using Content.Shared.Movement.Events;
 using Content.Shared.Movement.Pulling.Events;
+using Content.Shared.Polymorph;
 using Content.Shared.Tag;
 using Content.Shared.Verbs;
 using Robust.Shared.Containers;
@@ -42,6 +43,7 @@ public override void Initialize()
         SubscribeLocalEvent<FollowedComponent, ComponentGetStateAttemptEvent>(OnFollowedAttempt);
         SubscribeLocalEvent<FollowerComponent, GotEquippedHandEvent>(OnGotEquippedHand);
         SubscribeLocalEvent<FollowedComponent, EntityTerminatingEvent>(OnFollowedTerminating);
+        SubscribeLocalEvent<FollowedComponent, PolymorphedEvent>(OnFollowedPolymorphed);
         SubscribeLocalEvent<BeforeSaveEvent>(OnBeforeSave);
     }
 
@@ -142,6 +144,15 @@ private void OnFollowedTerminating(EntityUid uid, FollowedComponent component, r
         StopAllFollowers(uid, component);
     }
 
+    private void OnFollowedPolymorphed(Entity<FollowedComponent> entity, ref PolymorphedEvent args)
+    {
+        foreach (var follower in entity.Comp.Following)
+        {
+            // Stop following the target's old entity and start following the new one
+            StartFollowingEntity(follower, args.NewEntity);
+        }
+    }
+
     /// <summary>
     ///     Makes an entity follow another entity, by parenting to it.
     /// </summary>

Content.Shared/Polymorph/PolymorphEvents.cs

@@ -0,0 +1,10 @@
+namespace Content.Shared.Polymorph;
+
+/// <summary>
+/// Raised locally on an entity when it polymorphs into another entity
+/// </summary>
+/// <param name="OldEntity">EntityUid of the entity before the polymorph</param>
+/// <param name="NewEntity">EntityUid of the entity after the polymorph</param>
+/// <param name="IsRevert">Whether this polymorph event was a revert back to the original entity</param>
+[ByRefEvent]
+public record struct PolymorphedEvent(EntityUid OldEntity, EntityUid NewEntity, bool IsRevert);