title: What can cause a HV000028 validation exception?
body: <p>The application can persist users which can be modified later. Lately a user could not be modified and the <code>HV000028</code> exception was thrown. The user entity was persisted without errors or validation. 
  Does someone have an idea what could have caused such a behaviour or how I could find out more details? </p>
  
  <pre><code>2018-06-29 13:40:29,612 INFO  [stdout] (default task-48) Caused by: javax.validation.ValidationException: HV000028: Unexpected exception during isValid call.
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.constraintvalidation.ConstraintTree.validateSingleConstraint(ConstraintTree.java:451)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.constraintvalidation.ConstraintTree.validateConstraints(ConstraintTree.java:127)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.constraintvalidation.ConstraintTree.validateConstraints(ConstraintTree.java:87)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.metadata.core.MetaConstraint.validateConstraint(MetaConstraint.java:73)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validateMetaConstraint(ValidatorImpl.java:592)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validateConstraint(ValidatorImpl.java:555)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validateConstraintsForDefaultGroup(ValidatorImpl.java:490)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validateConstraintsForCurrentGroup(ValidatorImpl.java:454)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validateInContext(ValidatorImpl.java:406)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.hibernate.validator.internal.engine.ValidatorImpl.validate(ValidatorImpl.java:204)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at org.springframework.validation.beanvalidation.SpringValidatorAdapter.validate(SpringValidatorAdapter.java:253)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at sun.reflect.GeneratedMethodAccessor427.invoke(Unknown Source)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  2018-06-29 13:40:29,612 INFO  [stdout] (default task-48)        at java.lang.reflect.Method.invoke(Method.java:498)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.proxy.LazyInitProxyFactory$JdkHandler.invoke(LazyInitProxyFactory.java:507)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at com.sun.proxy.$Proxy287.validate(Unknown Source)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at ch.lmv.ulm.web.page.template.BasePanel.doCompleteJSR303Validation(BasePanel.java:65)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at ch.lmv.ulm.web.page.template.BasePanel.doCompleteJSR303Validation(BasePanel.java:49)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at ch.lmv.ulm.web.page.person.EditPersonPanel$7.onSubmit(EditPersonPanel.java:420)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.ajax.markup.html.form.AjaxSubmitLink$1.onSubmit(AjaxSubmitLink.java:110)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.ajax.form.AjaxFormSubmitBehavior$AjaxFormSubmitter.onSubmit(AjaxFormSubmitBehavior.java:215)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.markup.html.form.Form.delegateSubmit(Form.java:1307)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.markup.html.form.Form.process(Form.java:974)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.markup.html.form.Form.onFormSubmitted(Form.java:795)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.ajax.form.AjaxFormSubmitBehavior.onEvent(AjaxFormSubmitBehavior.java:171)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:155)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:588)
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48)        ... 82 more
  2018-06-29 13:40:29,613 INFO  [stdout] (default task-48) Caused by: java.lang.NullPointerException
  </code></pre>