ireport获取不到列的对象

[一手遮天~~]

//获取ColumnHeader拦
			JRDesignBand columnHeaderBand = (JRDesignBand)jasperDesign.getColumnHeader();
			
			JRDesignBand detailBand = (JRDesignBand )jasperDesign.getDetailSection().getBands()[0];
			
			JRElement[] es_header = columnHeaderBand.getElements();
					
			JRDesignStaticText columnHeaderField = (JRDesignStaticText)columnHeaderBand.getElementByKey("aa");
			
			JRDesignTextField detailField= (JRDesignTextField )detailBand.getElementByKey("ss");
			
			
	        columnHeaderBand.removeElement(columnHeaderField);
	        
            detailBand.removeElement(detailField);//得到对象之后  将其从xml的文件中移除

columnHeaderField 和detailField都为空，导致删除和添加列报错，getElementByKey获取的Key是列的字段名吗？现在获取字段名出现null，有哪位遇到过，求解决