<求助>java.lang.NullPointerException
错误页面：

Struts Problem Report
Struts has detected an unhandled exception: 

Messages:  
File: willow/business/code/action/CodeAction.java 
Line number: 28 


--------------------------------------------

Stacktraces
java.lang.NullPointerException 
  willow.business.code.action.CodeAction.execute(CodeAction.java:28)
  sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
  sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
  java.lang.reflect.Method.invoke(Unknown Source)
  com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:453)
  com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:292)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:255)
  org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
  com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
  org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
  com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
  com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:211)
  com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
  com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:249)
  com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept
。。。。省略  
相关代码：

Java code

package willow.business.code.action;

import java.util.List;

import org.apache.log4j.Logger;
import org.springframework.beans.factory.BeanFactory;

import willow.business.code.CodeForm;
import willow.data.code.ICodeData;
import willow.entity.code.Code;

/**
 * Filename: CodeAction.java
 */
public class CodeAction extends CodeForm {

    private static fin