Multiplayer: Fix entity de-registering (#765)

This fixes: thread 'Compute Task Pool (4)' panicked at 'called
`Option::unwrap()` on a `None` value',
crates/multiplayer/src/playermsg.rs:270:78

Which was due to receiving a deregister message after the player left
the game. This can happen as a race-condition just after game end or if
the other player is kicked out due to network issues (but sends a
message around the time of the kick).

Fixes #757.

crates/multiplayer/src/playermsg.rs

@@ -209,7 +209,7 @@ impl<'w> NetEntityCommands<'w> {
         self.map.register(remote, local)
     }
 
-    fn deregister(&mut self, remote: EntityNet) -> Entity {
+    fn deregister(&mut self, remote: EntityNet) -> Option<Entity> {
         self.map.deregister(remote)
     }
 
@@ -261,16 +261,18 @@ impl EntityIdMapRes {
 
     /// De-registers an existing remote entity.
     ///
+    /// Returns None if the player does no longer exist.
+    ///
     /// See [`Self::register`].
     ///
     /// # Panics
     ///
     /// Panics if the entity is not registered.
-    fn deregister(&mut self, remote: EntityNet) -> Entity {
-        let player_entities = self.remote_to_local.get_mut(&remote.player()).unwrap();
+    fn deregister(&mut self, remote: EntityNet) -> Option<Entity> {
+        let player_entities = self.remote_to_local.get_mut(&remote.player())?;
         let local = player_entities.remove(remote.index()).unwrap();
         self.local_to_remote.remove(&local).unwrap();
-        local
+        Some(local)
     }
 
     /// Translates local entity ID to a remote entity ID in case the entity is
@@ -374,8 +376,9 @@ fn recv_messages(
                 ));
             }
             ToPlayers::Despawn { entity } => {
-                let local = net_commands.deregister(*entity);
-                despawn_events.send(NetRecvDespawnActiveEvent::new(local));
+                if let Some(local) = net_commands.deregister(*entity) {
+                    despawn_events.send(NetRecvDespawnActiveEvent::new(local));
+                }
             }
             ToPlayers::SetPath { entity, waypoints } => {
                 let Some(local) = net_commands.remote_local_id(*entity) else {