自定义渲染器上的JTable扼流圈

我有一个自定义渲染器 JTable . 当我 JTHT 显示器,我得到一个 NullPointerException 在 JTable.prepareRenderer() . 我不确定自定义渲染器的哪个部分不可准备,但我确定这是我的错。

渲染器的压缩版本(完整版本位于 this spot ):

public class GradeRenderer extends JLabel implements TableCellRenderer {    
    public Component getTableCellRendererComponent(JTable table, Object grade,
            boolean isSelected, boolean hasFocus, int row, int column) {
        if (grade == null) return new JLabel();
        Grade myself = (Grade) grade;
        int score = myself.getScore();
        setText(String.valueOf(score));
        Assessment thing = myself.getThing();
        GradeStatus status = myself.getStatus();
        AssessmentType type = thing.getType();
        // do some work with status
        // do some work with type
        return this;
    }
}

添加渲染器的代码(完整版本 here ):

model = new GradeEntryModel();
table = new JTable(model);
// some work with table
table.setDefaultRenderer(Grade.class, new GradeRenderer(true));

我哪里出了错,有什么线索吗?