线程中传递数组时抛出空指针异常问题

在写一个排序算法演示程序；先在主窗体中的Listener中调用engine：

private class StartListener implements ActionListener{
        public void actionPerformed(ActionEvent e){

            
            int[] unsort=UnsortGenerator();  //函数返回给unsort一个数组
            
            
            engine.setArray(unsort);
            engine.setAlgorithm(getAlgorithm());
            engine.setSleepTime(jslSpeed.getValue()*50);
            engine.setHistogram(histogram);  //Histogram是用来画柱状图的
            engine.run();
        }
    }

然后在Engine中：

private SelectionSort selectionSort=new SelectionSort();
...
if (thread != null && thread.getState() != Thread.State.TERMINATED)
                return;
            
            thread=new Thread(selectionSort);
            selectionSort.setArray(unsort);
            thread.start();

在SelectionSort中：

public class SelectionSort implements Runnable{
    private int[] unsort;
    private Histogram histogram;
    private int sleepTime;
    
    public void run(){
        selectionSort();
    }

    public void setArray(int[] unsort){
        this.unsort=unsort;
    }
    
    private void selectionSort(){
        
        int key=0;
        int count=0;
        while(count<unsort.length-1){
            for(int i=key+1;i<unsort.length;i++){
                if(unsort[key]>unsort[i])
                    key=i;
            }    
            int temp=unsort[count];
            unsort[count]=unsort[key];
            unsort[key]=temp;
            
            count++;
            key=count;
            
            for(int i=0;i<unsort.length;i++)
                System.out.printf("%d ",unsort[i]);
            System.out.print('\n');//这里可以输出正常数组

            histogram.showHistogram(unsort);//这里会抛出异常

            try {
                Thread.sleep(sleepTime);
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }
    }
}

请问这段代码哪里有问题？如何修改？