Attempted to call method length on null context object和“arrays.length(tag.blogs)“ (template: “tags“）

使用thymeelaf和springboot出现的bug，报错信息如下

org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/tags.html]")
	at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:241) ~[thymeleaf-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parseStandalone(AbstractMarkupTemplateParser.java:100) ~[thymeleaf-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.engine.TemplateManager.parseAndProcess(TemplateManager.java:667) ~[thymeleaf-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1087) [thymeleaf-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.TemplateEngine.process(TemplateEngine.java:1061) [thymeleaf-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.spring4.view.ThymeleafView.renderFragment(ThymeleafView.java:335) [thymeleaf-spring4-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.thymeleaf.spring4.view.ThymeleafView.render(ThymeleafView.java:189) [thymeleaf-spring4-3.0.2.RELEASE.jar:3.0.2.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1286) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.processDispatchResult(DispatcherServlet.java:1041) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:984) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:861) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) [tomcat-embed-core-8.5.20.jar:8.5.20]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) [spring-webmvc-4.3.11.RELEASE.jar:4.3.11.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) [tomcat-embed-core-8.5.20.jar:8.5.20]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) [tomcat-embed-core-8.5.20.jar:8.5.20]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) [tomcat-embed-core-8.5.20.jar:8.5.20]
	at org.apache.tomcat.websocket.server.WsFilter