[Client thread/FATAL]: Error executing task - FORGE 1.8.9

Good evening!

I tried to implement scheduled(delayed) tasks in my code and now every few seconds I get this message in terminal. I am using eclipse. The scheduled task seems to work fine I am just scared that this mod won't make too much for people using it.

If anyone could figure out 

Initializing(if this is correct term) the timer:

private final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);

The delayed function:

        void DelayedTask() {
         		//flag=false;
        	System.out.println("CHECK1");
        		if(DntbMod.DoQuests) {
        			m.thePlayer.sendChatMessage(GetGame());
        		}
        		else{m.thePlayer.sendChatMessage(DntbMod.komanda);}
        	}

Calling the delayed function:

sleep= 3000+(long)(Math.random()*5000);	   
scheduler.schedule(new Runnable() {public void run() { DelayedTask(); }}, sleep, MILLISECONDS);

Thank you for your help.

Erol