Null pointer exception clicking help icon next to fields on edit issue screen

Clicking one of the help icons next to an editable field on the edit issue screen produced the following exception:

An error occurred whilst rendering this message. Please contact the administrators, and inform them of this bug. Details: ------- java.lang.NullPointerException at org.apache.velocity.context.InternalContextAdapterImpl.get(InternalContextAdapterImpl.java:162) at org.apache.velocity.runtime.parser.node.ASTReference.getVariableValue(ASTReference.java:623) at org.apache.velocity.runtime.parser.node.ASTReference.execute(ASTReference.java:151) at org.apache.velocity.runtime.parser.node.ASTReference.render(ASTReference.java:220) at org.apache.velocity.runtime.parser.node.SimpleNode.render(SimpleNode.java:230) at org.apache.velocity.Template.merge(Template.java:256) at org.apache.velocity.app.VelocityEngine.mergeTemplate(VelocityEngine.java:450) at org.apache.velocity.app.VelocityEngine.mergeTemplate(VelocityEngine.java:419) at com.atlassian.velocity.DefaultVelocityManager.getEncodedBody(DefaultVelocityManager.java:74) at com.atlassian.jira.plugin.webwork.JiraPluginWebworkVelocityServlet.service(JiraPluginWebworkVelocityServlet.java:50) at javax.servlet.http.HttpServlet.service(HttpServlet.java:802) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173) at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672) at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:463) at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:398) at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:301) at com.atlassian.jira.web.dispatcher.JiraServletDispatcher.service(JiraServletDispatcher.java:248) at