nullエラーを解決したい...

長文失礼します。

Java(Eclipse)でタスク管理アプリ(Index,Create,Show,Edit)を作成をしています。

コード上はエラーはないのですが、ブラウザ上で動作確認すると500エラー(null)になってしまいます。

今現在、IndexとNewは動く状態です。

エラーの内容です。
java.lang.NumberFormatException: null
java.lang.Integer.parseInt(Integer.java:542)
java.lang.Integer.parseInt(Integer.java:615)
controllers.editServlet.doGet(editServlet.java:37)
javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
filters.EncodingFilter.doFilter(EncodingFilter.java:38)

初心者のため、この部分がこうなってるからこう書き換えるという感じで教えていただくと嬉しいです。
よろしくお願いします。
Macbook Air使用。Java8とTomcat8使用です。

１枚目(edit.jsp)
２枚目(editServlet)
３枚目(show.jsp)
４枚目(showServlet)
５枚目(index.jsp)
６枚目(indexServlet)
７枚目(new.jsp)
８枚目(newServlet)

edit.jsp

java
1<%@ page language="java" contentType="text/html; charset=UTF-8"
2    pageEncoding="UTF-8"%>
3<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
4<c:import url="../layout/app.jsp">
5    <c:param name="content">
6        <h2>id : ${task.id}の編集ページ</h2>
7        <form method="POST" action="${pageContext.request.contextPath}/update">
8            <c:import url="_form.jsp" />
9        </form>
10        <p>
11            <a href="${pageContext.request.contextPath}/index">一覧に戻る</a>
12        </p>
13        <p>
14            <a href="#" onclick="confirmDestroy();">削除</a>
15        </p>
16        <form method="POST"
17            action="${pageContext.request.contextPath}/destroy">
18            <input type="hidden" name="_token" value="${_token}" />
19        </form>
20        <script>
21            function confirmDestroy() {
22                if (confirm("本当に削除しますか？")) {
23                    document.forms[1].submit();
24                }
25            }
26        </script>
27
28    </c:param>
29</c:import>
30

editServlet

java
1package controllers;
2
3import java.io.IOException;
4
5import javax.persistence.EntityManager;
6import javax.servlet.RequestDispatcher;
7import javax.servlet.ServletException;
8import javax.servlet.annotation.WebServlet;
9import javax.servlet.http.HttpServlet;
10import javax.servlet.http.HttpServletRequest;
11import javax.servlet.http.HttpServletResponse;
12
13import models.TaskData;
14import utils.DBUtil;
15
16/**
17 * Servlet implementation class EditServlet
18 */
19@WebServlet("/edit")
20public class editServlet extends HttpServlet {
21    private static final long serialVersionUID = 1L;
22
23    /**
24     * @see HttpServlet#HttpServlet()
25     */
26    public editServlet() {
27        super();
28    }
29
30    /**
31     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
32     *      response)
33     */
34    protected void doGet(HttpServletRequest request, HttpServletResponse response)
35            throws ServletException, IOException {
36        EntityManager em = DBUtil.createEntityManager();
37        TaskData t = em.find(TaskData.class, Integer.parseInt(request.getParameter("id")));
38        em.close();
39        request.setAttribute("task", t);
40        request.setAttribute("_token", request.getSession().getId());
41        request.getSession().setAttribute("task_id", t.getId());
42        RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/tasks/edit.jsp");
43        rd.forward(request, response);
44    }
45}
46```show.jsp
47```java
48<%@ page language="java" contentType="text/html; charset=UTF-8"
49    pageEncoding="UTF-8"%>
50<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
51<%@ taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt"%>
52<c:import url="../layout/app.jsp">
53    <c:param name="content">
54        <h2>id : ${task.id} 詳細</h2>
55        <p>
56            <c:out value="${task.content}" />
57        </p>
58        <p>
59            作成日時
60            <fmt:formatDate value="${task.created_at}"
61                pattern="yyyy-MM-dd HH:mm:ss" />
62        </p>
63        <p>
64            更新日時
65            <fmt:formatDate value="${task.updated_at}"
66                pattern="yyyy-MM-dd HH:mm:ss" />
67        </p>
68        <p>
69            <a href="${pageContext.request.contextPath}/index">一覧にもどる</a>
70        </p>
71        <p><a href="${pageContext.request.contextPath}/edit?id=${message.id}">編集</a></p>
72    </c:param>
73</c:import>

showServlet

java
1package controllers;
2
3import java.io.IOException;
4
5import javax.persistence.EntityManager;
6import javax.servlet.RequestDispatcher;
7import javax.servlet.ServletException;
8import javax.servlet.annotation.WebServlet;
9import javax.servlet.http.HttpServlet;
10import javax.servlet.http.HttpServletRequest;
11import javax.servlet.http.HttpServletResponse;
12
13import models.TaskData;
14import utils.DBUtil;
15
16/**
17 * Servlet implementation class ShowServlet
18 */
19@WebServlet("/show")
20public class ShowServlet extends HttpServlet {
21    private static final long serialVersionUID = 1L;
22
23    /**
24     * @see HttpServlet#HttpServlet()
25     */
26    public ShowServlet() {
27        super();
28        // TODO Auto-generated constructor stub
29    }
30
31    /**
32     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
33     *      response)
34     */
35    protected void doGet(HttpServletRequest request, HttpServletResponse response)
36            throws ServletException, IOException {
37        EntityManager em = DBUtil.createEntityManager();
38        TaskData t = em.find(TaskData.class, Integer.parseInt(request.getParameter("id")));
39        em.close();
40        request.setAttribute("task", t);
41        RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/tasks/show.jsp");
42        rd.forward(request, response);
43    }
44}
45```index.jsp
46```java
47<%@ page language="java" contentType="text/html; charset=UTF-8"
48    pageEncoding="UTF-8"%>
49<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
50<c:import url="../layout/app.jsp">
51    <c:param name="content">
52        <h2>タスク一覧</h2>
53        <ul>
54            <c:forEach var="task" items="${tasks}">
55                <li>
56                    <a href="${pageContext.request.contextPath}/show?id=${task.id}">
57                        <c:out value="${task.id}" />
58                    </a> <c:out value="${task.content}" />
59                </li>
60            </c:forEach>
61        </ul>
62        <p>
63            <a href="${pageContext.request.contextPath}/new">新規タスク</a>
64        </p>
65    </c:param>
66</c:import>
67
68```indexServlet
69```java
70package controllers;
71
72import java.io.IOException;
73import java.util.List;
74
75import javax.persistence.EntityManager;
76import javax.servlet.RequestDispatcher;
77import javax.servlet.ServletException;
78import javax.servlet.annotation.WebServlet;
79import javax.servlet.http.HttpServlet;
80import javax.servlet.http.HttpServletRequest;
81import javax.servlet.http.HttpServletResponse;
82
83import models.TaskData;
84import utils.DBUtil;
85
86/**
87 * Servlet implementation class IndexServlet
88 */
89@WebServlet("/index")
90public class indexServlet extends HttpServlet {
91    private static final long serialVersionUID = 1L;
92
93    /**
94     * @see HttpServlet#HttpServlet()
95     */
96    public indexServlet() {
97        super();
98        // TODO Auto-generated constructor stub
99    }
100
101    /**
102     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
103     *      response)
104     */
105    protected void doGet(HttpServletRequest request, HttpServletResponse response)
106            throws ServletException, IOException {
107        EntityManager em = DBUtil.createEntityManager();
108        List<TaskData> tasks = em.createNamedQuery("getAllTasks", TaskData.class).getResultList();
109        response.getWriter().append(Integer.valueOf(tasks.size()).toString());
110        em.close();
111        request.setAttribute("tasks", tasks);
112
113        RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/tasks/index.jsp");
114        rd.forward(request, response);
115
116    }
117
118}
119```new.jsp
120```java
121<%@ page language="java" contentType="text/html; charset=UTF-8"
122    pageEncoding="UTF-8"%>
123<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
124<c:import url="../layout/app.jsp">
125    <c:param name="content">
126        <h2>新規作成</h2>
127        <form method="POST" action="${pageContext.request.contextPath}/create">
128            <c:import url="_form.jsp" />
129        </form>
130        <p>
131            <a href="${pageContext.request.contextPath}/index">一覧に戻る</a>
132        </p>
133    </c:param>
134</c:import>

newServlet

java
1package controllers;
2
3import java.io.IOException;
4
5import javax.servlet.RequestDispatcher;
6import javax.servlet.ServletException;
7import javax.servlet.annotation.WebServlet;
8import javax.servlet.http.HttpServlet;
9import javax.servlet.http.HttpServletRequest;
10import javax.servlet.http.HttpServletResponse;
11
12import models.TaskData;
13
14/**
15 * Servlet implementation class NewServlet
16 */
17@WebServlet("/new")
18public class NewServlet extends HttpServlet {
19    private static final long serialVersionUID = 1L;
20
21    /**
22     * @see HttpServlet#HttpServlet()
23     */
24    public NewServlet() {
25        super();
26        // TODO Auto-generated constructor stub
27    }
28
29    /**
30     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse
31     *      response)
32     */
33    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
34        // CSRF�΍�
35         request.setAttribute("_token", request.getSession().getId());
36         // ����
37         request.setAttribute("task", new TaskData());
38         RequestDispatcher rd = request.getRequestDispatcher("/WEB-INF/views/tasks/new.jsp");
39         rd.forward(request, response);
40
41    }
42
43}
44