having problem with protocolManager is null

[khangdh5200]

1. I am creating plugin in version 1.20.4 paper (bukkit),
2. version dependencies:
   
   com.comphenix.protocol
   ProtocolLib
   5.1.0
   
3. I added ProtocolLib.jar to my library, the specific version is v5.2.0-SNAPSHOT-679

4. Here my code:

 private ProtocolManager protocolManager;
     @Override
     public void onEnable() {
         if(getServer().getPluginManager().getPlugin("ProtocolLib") == null) {
             getLogger().severe("ProtocolLib not found");
             getServer().getPluginManager().disablePlugin(this);
             return;
         }
         protocolManager = ProtocolLibrary.getProtocolManager();
         protocolManager.addPacketListener(new PacketAdapter(this, PacketType.Handshake.Client.SET_PROTOCOL) {
             @Override
             public void onPacketReceiving(PacketEvent event) {
                 if(event.getPacketType() == PacketType.Handshake.Client.SET_PROTOCOL) {
                     int clientVersion = event.getPacket().getIntegers().read(0);
                     getLogger().info("Player " + event.getPlayer().getName() + " is using client version: " + clientVersion);
                 }
             }
         });
     }

5. This is an error on the console:

 Error occurred while enabling TempProtocol v1.0-Beta (Is it up to date?)
java.lang.NullPointerException: Cannot invoke "com.comphenix.protocol.ProtocolManager.addPacketListener(com.comphenix.protocol.events.PacketListener)" because "this.protocolManager" is null
        at kanci.gamer.tempprotocol.TempProtocol.onEnable(TempProtocol.java:31) ~[TempProtocol-1.0-Beta.jar:?]
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:287) ~[paper-api-1.20.4-R0.1-SNAPSHOT.jar:?]
        at io.papermc.paper.plugin.manager.PaperPluginInstanceManager.enablePlugin(PaperPluginInstanceManager.java:188) ~[paper-1.20.4.jar:git-Paper-496]

6. This is the line that reports the error in the code, as I understand it:

protocolManager.addPacketListener(new PacketAdapter(this, PacketType.Handshake.Client.SET_PROTOCOL) {}

Please guide me how to fix this error. I'm new to programming minecraft plugins, please guide me. Thank people!

[khangdh5200]

Because I'm trying to connect dependencies with Protocol, my code is just that simple, it has all the libraries added (it doesn't error libraries or codes). I just declare a ProtocolManager and run it in onEnable.