Account for null servers in bungee

bungeecord/src/main/java/io/github/retrooper/packetevents/bungee/factory/BungeePacketEventsBuilder.java

@@ -46,6 +46,7 @@
 import net.kyori.adventure.text.format.NamedTextColor;
 import net.md_5.bungee.api.ProxyServer;
 import net.md_5.bungee.api.connection.ProxiedPlayer;
+import net.md_5.bungee.api.connection.Server;
 import net.md_5.bungee.api.plugin.Plugin;
 import net.md_5.bungee.protocol.ProtocolConstants;
 import org.jetbrains.annotations.NotNull;
@@ -109,7 +110,17 @@ public ServerVersion getVersion() {
                 @Override
                 public Object getRegistryCacheKey(User user, ClientVersion version) {
                     ProxiedPlayer player = ProxyServer.getInstance().getPlayer(user.getUUID());
-                    return player == null ? null : Objects.hash(player.getServer().getInfo(), version);
+                    if (player == null) {
+                        return null;
+                    }
+                    Server server = player.getServer();
+                    if (server == null) {
+                        // seems to be null during server switch or on join,
+                        // but only for some specific bungee forks?
+                        // BungeeCord would be a lot safer if they were to use nullability annotations...
+                        return null;
+                    }
+                    return Objects.hash(server.getInfo(), version);
                 }
             };