Drawing a ListField and painting the screen with Graphics based on a vector parsing information from an XML file

   public class CategoriesUI extends MainScreen implements ListFieldCallback {

    //categoryimport.listingnodup is the current categories with no duplicates
    public Categories categoryimport = new Categories(); //brings in all infromation from Categories.java

    private ListField allcategories;

    CategoriesUI() {     

        this.add(new LabelField("List of Categories"));
        allcategories = new ListField(categoryimport.listingnodup.size());
        allcategories.setCallback(this); //we manage the interaction!
        this.add(allcategories);

    }

    protected boolean onSavePrompt() {

       开发者_StackOverflow社区 return true;
    }


    //Implemented Call Back Methods follow

    //draw the current row
    public void drawListRow(ListField list, Graphics g, int index, int y, int w) {

        catdrawer categorydraw = (catdrawer) this.get(list, index);
        int drawColor = Color.BLACK;
        g.setColor(drawColor);
        g.drawText(categorydraw.cat, 0, y, 0, w);

    }

    public int getPreferredWidth(ListField list) {

        return Display.getWidth();

    }   

    public int indexOfList(ListField listField, String prefix, int start) {

        //Not a current implementation this is really just commented out
        return start;

    }

    //Returns the object at the specified index
    public Object get(ListField list, int index){

        return categoryimport.listingnodup.elementAt(index);

    }

    class catdrawer {

        public String cat = categoryimport.listingnodup.toString(); 

    }    
    }

The program complies correctly but when it runs in the Simulator 8800 it crashes it when this code is executed.

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This code is the source of the problem:

catdrawer categorydraw = (catdrawer) this.get(list, index);

When categorydraw is null, the call to drawtext 3 lines later will throw an exception. You need to check for null.