NPE in WebResourceManager when attaching file

When attempting to attach a file on s.a.c I experienced a delay, then the error below. Trying the same attachment again a few seconds later worked.

500 Servlet Exception
java.lang.NullPointerException
at com.atlassian.jira.plugin.webresource.JiraWebResourceIntegration.getBaseUrl(JiraWebResourceIntegration.java:42)
at com.atlassian.plugin.webresource.WebResourceManagerImpl.getStaticResourcePrefix(WebResourceManagerImpl.java:182)
at jsp._500page_jsp._jspService(includes/decorators/stylesheettag.jsp:12)
at com.caucho.jsp.JavaPage.service(JavaPage.java:60)
at com.caucho.jsp.Page.pageservice(Page.java:570)
at com.caucho.server.dispatch.PageFilterChain.doFilter(PageFilterChain.java:175)
at com.caucho.server.webapp.DispatchFilterChain.doFilter(DispatchFilterChain.java:115)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:229)
at com.caucho.server.webapp.RequestDispatcherImpl.forward(RequestDispatcherImpl.java:277)
at com.caucho.server.webapp.RequestDispatcherImpl.error(RequestDispatcherImpl.java:113)
at com.caucho.server.webapp.ErrorPageManager.sendServletError(ErrorPageManager.java:363)
at com.caucho.server.webapp.WebAppFilterChain.doFilter(WebAppFilterChain.java:180)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:229)
at com.caucho.server.hmux.HmuxRequest.handleRequest(HmuxRequest.java:419)
at com.caucho.server.port.TcpConnection.run(TcpConnection.java:389)
at com.caucho.util.ThreadPool.runTasks(ThreadPool.java:507)
at com.caucho.util.ThreadPool.run(ThreadPool.java:433)
at java.lang.Thread.run(Thread.java:595)