Call another .java [duplicate]

I'm trying to create a discord bot with java maven and I would like to have the core separated from the commands, and that the core call (start) the commands so that they can be used, I tried to call it in the following way but not it works, it gives me an error of java.lang.NullPoinerException I would appreciate someone helping me:

THE NUCLEUS

package darkdragon.team.dragonoid;

import net.dv8tion.jda.core.AccountType;
import net.dv8tion.jda.core.JDA;
import net.dv8tion.jda.core.JDABuilder;
import net.dv8tion.jda.core.entities.Game;
import net.dv8tion.jda.core.events.message.MessageReceivedEvent;
import net.dv8tion.jda.core.hooks.ListenerAdapter;

public class App extends ListenerAdapter
{
    public static JDA api;
    public static void main( String[] args ) throws Exception
    {
        api = new JDABuilder(AccountType.BOT).setToken(Ref.token).buildBlocking();
        api.addEventListener(new App());
        api.getPresence().setGame(Game.playing("MANTENIMIENTO"));
        MessageReceivedEvent evt = null;
        Commands.processCommand(evt);
    }

}

THE COMMANDS

package darkdragon.team.dragonoid;

import net.dv8tion.jda.core.entities.Message;
import net.dv8tion.jda.core.entities.MessageChannel;
import net.dv8tion.jda.core.entities.User;
import net.dv8tion.jda.core.events.message.MessageReceivedEvent;
import net.dv8tion.jda.core.hooks.ListenerAdapter;

public class Commands extends ListenerAdapter {
    public static void processCommand(MessageReceivedEvent evt) {
        User objUser = evt.getAuthor();
        Message objMsg = evt.getMessage();
        MessageChannel objMsgCh = evt.getChannel();
        String[] command = objMsg.getContentRaw().toLowerCase().replaceFirst(Ref.prefix, "").split(" ");
        if(command[0].equals("ping")) {
            objMsgCh.sendMessage(objUser.getAsMention() + "Pong!");
        }
    }
}

SKIP THE ERROR

  

Exception in thread "main" java.lang.NullPointerException