書籍
Spring解体新書第2版
6章実施中

環境
・Windows10
・STS
・Spring boot
・Tymeleafテンプレートエンジン

発生したエラー

There was an unexpected error (type=Internal Server Error, status=500).
An error happened during template parsing (template: "class path resource [templates/user/signup.html]")
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/user/signup.html]")
	at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241)
	at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100)
	at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:666)
	at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1103)
	at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1077)
	at org.thymeleaf.spring6.view.ThymeleafView.renderFragment(ThymeleafView.java:372)
	at org.thymeleaf.spring6.view.ThymeleafView.render(ThymeleafView.java:192)
	at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1432)
	at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1168)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1106)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014)
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:903)
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:564)
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:885)
	at jakarta.servlet.http.HttpServlet.service(HttpServlet.java:658)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:195)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:140)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:164)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:140)
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:164)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:140)
	at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:164)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:140)
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:116)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:164)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:140)
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:167)
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:90)
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:483)
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:115)
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:93)
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:344)
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:384)
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:63)
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:904)
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1741)
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:52)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1190)
	at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659)
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:63)
	at java.base/java.lang.Thread.run(Thread.java:1583)
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "item.value" (template: "user/signup" - line 48, col 51)
	at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393)
	at org.attoparser.MarkupParser.parse(MarkupParser.java:257)
	at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230)
	... 48 more
Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "item.value" (template: "user/signup" - line 48, col 51)
	at org.thymeleaf.spring6.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:292)
	at org.thymeleaf.standard.expression.VariableExpression.executeVariableExpression(VariableExpression.java:166)
	at org.thymeleaf.standard.expression.SimpleExpression.executeSimple(SimpleExpression.java:66)
	at org.thymeleaf.standard.expression.Expression.execute(Expression.java:109)
	at org.thymeleaf.standard.expression.Expression.execute(Expression.java:138)
	at org.thymeleaf.standard.processor.AbstractStandardExpressionAttributeTagProcessor.doProcess(AbstractStandardExpressionAttributeTagProcessor.java:144)
	at org.thymeleaf.processor.element.AbstractAttributeTagProcessor.doProcess(AbstractAttributeTagProcessor.java:74)
	at org.thymeleaf.processor.element.AbstractElementTagProcessor.process(AbstractElementTagProcessor.java:95)
	at org.thymeleaf.util.ProcessorConfigurationUtils$ElementTagProcessorWrapper.process(ProcessorConfigurationUtils.java:633)
	at org.thymeleaf.engine.ProcessorTemplateHandler.handleStandaloneElement(ProcessorTemplateHandler.java:918)
	at org.thymeleaf.engine.TemplateHandlerAdapterMarkupHandler.handleStandaloneElementEnd(TemplateHandlerAdapterMarkupHandler.java:260)
	at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler$InlineMarkupAdapterPreProcessorHandler.handleStandaloneElementEnd(InlinedOutputExpressionMarkupHandler.java:256)
	at org.thymeleaf.standard.inline.OutputExpressionInlinePreProcessorHandler.handleStandaloneElementEnd(OutputExpressionInlinePreProcessorHandler.java:169)
	at org.thymeleaf.templateparser.markup.InlinedOutputExpressionMarkupHandler.handleStandaloneElementEnd(InlinedOutputExpressionMarkupHandler.java:104)
	at org.attoparser.HtmlVoidElement.handleOpenElementEnd(HtmlVoidElement.java:92)
	at org.attoparser.HtmlMarkupHandler.handleOpenElementEnd(HtmlMarkupHandler.java:297)
	at org.attoparser.MarkupEventProcessorHandler.handleOpenElementEnd(MarkupEventProcessorHandler.java:402)
	at org.attoparser.ParsingElementMarkupUtil.parseOpenElement(ParsingElementMarkupUtil.java:159)
	at org.attoparser.MarkupParser.parseBuffer(MarkupParser.java:710)
	at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:301)
	... 50 more
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'value' cannot be found on null
	at org.springframework.expression.spel.ast.PropertyOrFieldReference.readProperty(PropertyOrFieldReference.java:225)
	at org.springframework.expression.spel.ast.PropertyOrFieldReference.getValueInternal(PropertyOrFieldReference.java:112)
	at org.springframework.expression.spel.ast.PropertyOrFieldReference$AccessorValueRef.getValue(PropertyOrFieldReference.java:417)
	at org.springframework.expression.spel.ast.CompoundExpression.getValueInternal(CompoundExpression.java:97)
	at org.springframework.expression.spel.ast.SpelNodeImpl.getValue(SpelNodeImpl.java:114)
	at org.springframework.expression.spel.standard.SpelExpression.getValue(SpelExpression.java:338)
	at org.thymeleaf.spring6.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:265)
	... 69 more

エラー見てもなにがなんだか、、という状態でした。
以下対象ファイルです。

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<meta>
<meta charset="UTF-8">
</meta>
<meta name="viewport" content="width=device-width, initail-scale=1, shrink-to-fit=no">
<!--CCS読込-->
<link rel="stylesheet" th:href="@{/webjars/bootstrap/css/bootstrap.min.css}">
<link rel="stylesheet" th:href="@{/css/login/login.css}">
<!--JS読込-->
<script th:src="@{webjars/jquery/jquery.min.js}" defer></script>
<script th.src="@{webjars/bootstrap/css/bootstrap.min.js}" defer></script>
<title>ユーザ登録</title>
</head>

<body class="bg-light">
	<div class="text-center">
		<form id="signup-form" method="post" action="/user/signup" class="form-signup">
			<h1 class="text-center">ユーザ登録</h1>
			<!--ユーザID-->
			<div class="form-group">
				<label for="userId">ユーザId</label>
				<input type="text" class="form-control">
			</div>
			<!--パスワード-->
			<div class="form-group">
				<label for="password">パスワード</label>
				<input type="text" class="form-control">
			</div>
			<!--ユーザー名-->
			<div class="form-group">
				<label for="userName">ユーザー名</label>
				<input type="text" class="form-control">
			</div>
			<!--誕生日-->
			<div class="form-group">
				<label for="birthday">誕生日</label>
				<input type="text" class="form-control">
			</div>
			<!--年齢-->
			<div class="form-group">
				<label for="age">年齢</label>
				<input type="text" class="form-control">
			</div>
			<!--性別-->
			<div class="form-group">
				<div ty:each="item : ${genderMap}" class="form-check-inline">
					<input type="radio" class="form-check-input" th:value="${item.value}">
					<label class="form-check-label" th:text="${item.key}"></label>
				</div>
			</div>
			<!--登録ボタン-->
			<input type="submit" value="ユーザ登録" class="btn btn-primary w-100 mt-3" />
			</div>
		</form>
</body>
</html>

参考にした投稿
https://qiita.com/mako0104/items/e24890a32f74c538f9e5

ページ内でlineを検索してみよう！
この記事を参照し、実施してみました。
そうすると該当しました。

Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "item.value" (template: "user/signup" - line 48, col 51)

見てみると「ん、、48行目といっているけど47行目なんか違うな、、」
ty:eachをth:eachに変更しました。

<div ty:each="item : ${genderMap}" class="form-check-inline">
	<input type="radio" class="form-check-input" th:value="${item.value}">
	<label class="form-check-label" th:text="${item.key}"></label>

新規登録画面が無事開けるようになりました。

エラー解消の流れ
・エラーログのページ内検索でline

エラー解決に手間取ったので、どなたかの解決案となれば幸いです。