I would use:
bw.write( model.getValueAt(i, j).toString() );
which will write the String representation of any Object you might be storiung in your TableModel.
Edit:
The NPE is caused by the bw.write(model.getValueAt(i,j).toString()); line
So what is null, "bw", "model", the data from the getValue(...) method?
I'm guessing the data, in which cause you can use code like:
Object data = model.getValueAt(I, j);
if (data == null)
System.out.println("null data at - " + I + " : " + j);
else
bw.write( data.toString() );
then once you know what cell(s) are null you investigate to find out why.