java 弹出下载框后不停的报空指针异常？

public String exPortTxt() {
if (form == null){
form = ServletActionContext.getRequest().getSession().getAttribute("form").toString();
System.out.println("TXT中form是"+form);
}
String sql = "";
if (form.equals("0")) {
sql = "select * from (select A.*, rownum RN from (select * from cc_compare_crbt) A )where RN > ? and RN<= ?";
} else if (form.equals("1")) {

sql = ServletActionContext.getRequest().getSession().getAttribute("sql").toString();
}

HttpServletResponse response = ServletActionContext.getResponse();
Map map = null;
List list = null;
try {
if(selectFile.trim().length()==0){
selectFile="1";
}
map = comparecrbtservice.exPortQuery(Integer.parseInt(selectFile), sql);
list = (List) map.get("list");
} catch (Exception e1) {

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