JBAS014612: NullPointerException from "read-attribute" management operation in EAP 6

Issue

I'm seeing the following error in my log:

JBAS014612: Operation ("read-attribute") failed - address: ([
    ("deployment" => "example.war"),
    ("subsystem" => "web"),
    ("servlet" => "testServlet")
]): java.lang.NullPointerException
    at org.jboss.as.web.WebDeploymentServletDefinition$5.handle(WebDeploymentServletDefinition.java:91)
    at org.jboss.as.web.WebDeploymentServletDefinition$AbstractMetricsHandler$1.execute(WebDeploymentServletDefinition.java:119)
    at org.jboss.as.controller.AbstractOperationContext.executeStep(AbstractOperationContext.java:611)
    at org.jboss.as.controller.AbstractOperationContext.doCompleteStep(AbstractOperationContext.java:489)
    at org.jboss.as.controller.AbstractOperationContext.completeStepInternal(AbstractOperationContext.java:290)

What does it mean, and how can I prevent it?