my code  i got error as null pointer exception

public class CatListAdapter extends ArrayAdapter {
ArrayList <String> alNames;
ArrayList <String> alImages;
ArrayList <String> alId;
private Activity context;
float cornerRadius = 50.0f;
ImageView imageFlag;

public CatListAdapter(Activity context,ArrayList <String> alNames,  ArrayList <String> alImages, ArrayList <String> alId) {
    super(context, R.layout.category_list, alNames);
    this.context = context;
    this.alNames = alNames;
    this.alImages = alImages;
    this.alId = alId;

}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View row=convertView;
    LayoutInflater inflater = context.getLayoutInflater();
    if(convertView==null)
        row = inflater.inflate(R.layout.category_list, null, true);
    TextView textViewCountry = (TextView) row.findViewById(R.id.txtcatname);

    imageFlag = (ImageView) row.findViewById(R.id.catimage);

    textViewCountry.setText(alNames.get(position));
    if (String.valueOf(alImages.get(position)) != "") {


        byte[] decode = Base64.decode(alImages.get(position), Base64.DEFAULT);
        Bitmap bitmap = BitmapFactory.decodeByteArray(decode, 0, decode.length);
        bitmapResize(bitmap);

    }



    return  row;
}
public void bitmapResize(Bitmap imageBitmap) {


    float widthbmp = imageBitmap.getWidth();
    float lengthbmp = imageBitmap.getHeight();
    // Get Screen width
    DisplayMetrics displaymetrics = new DisplayMetrics();
    context.getWindowManager().getDefaultDisplay().getMetrics(displaymetrics);
    float hight = displaymetrics.heightPixels / 3;
    float width = displaymetrics.widthPixels / 3;

    int convertHighet = (int) hight, convertWidth = (int) width;

    // high length
    if (lengthbmp > hight) {
        convertHighet = (int) hight - 20;
        imageBitmap = Bitmap.createScaledBitmap(imageBitmap, convertWidth,
                convertHighet, true);
    }

    // high width
    if (widthbmp > width) {
        convertWidth = (int) width - 20;
        imageBitmap = Bitmap.createScaledBitmap(imageBitmap, convertWidth,
                convertHighet, true);
        imageFlag.setImageBitmap(imageBitmap);
    }


}