nullpointer exception in array list

public String tornadoComing()
{
    int open= 0;
    int closed= 0;
    for (int i= 0; i< doorsHave.length; i++)
    {
        doorsHave[i].open();
    }
    for (int i=0; i<doorsNeed.size();i++)
    {
        if (doorsNeed.get(i).getState()== false)
        {
            doorsNeed.remove(doorsNeed.get(i));
            closed++;
        }
        else 
        {
            open++;
        }
    }
    return open + " doors opened and " + closed + " doors closed";
} 

I am uncertain why this is throwing a null pointer.