Java一个线程中传递数组时抛出空指针异常

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？
提前感谢！