Erro: Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException:
at Dados.SimulaGui.jButtonParaConvActionPerformed(SimulaGui.java:408)

sistema.pauseT();

public class Sistema extends Thread {
public Autocarro autocarro;
public volatile boolean fim = false;
public JTextArea area;
public final Object autocarrox = new Object();
public boolean pauseThreadFlag = false;
 

public Sistema(JTextArea area) {
    super();
    this.area=area;
}

public Sistema(Autocarro autocarro)
{
    this.autocarro = autocarro;
}

//
public void run()
{
    while (!fim)
    {
        try 
        {
            sleep(Viagem.Espera * 1000);
            autocarro.andar();    
        }
        catch (InterruptedException ex)
        {
            ex.printStackTrace();
            return;
        }
       
    }            
}

//para terminar a thread.
public void fim()
{
    fim = true;
}


public void checkForPaused() {
    synchronized (autocarro) {
        while (fim) {
            try {
                autocarro.wait();
            } catch (Exception e) {}
        }
    }
}

public void pauseT() throws InterruptedException {
    fim = true;
}

public void resumeT() {
    synchronized(autocarro) {
        fim = false;
        autocarro.notify();
    }
}