why is hashcode() called on page deserialization?

[Martin Asenov]

Hello, everyone!

I get the exception:

java.lang.NullPointerException
     at com.company.project.event.SMSEvent.hashCode(SMSEvent.java:334)
     at java.util.HashMap.putForCreate(HashMap.java:413)
     at java.util.HashMap.readObject(HashMap.java:1031)
     at java.lang.reflect.Method.invoke(Method.java:597)

      .............

     at org.apache.wicket.Component.readObject(Component.java:4465)

      .............

     at org.apache.wicket.util.lang.Objects.byteArrayToObject(Objects.java:409)
     at 
org.apache.wicket.protocol.http.pagestore.AbstractPageStore.deserializePage(AbstractPageStore.java:234)
     at 
org.apache.wicket.protocol.http.pagestore.DiskPageStore.getPage(DiskPageStore.java:735)
     at 
org.apache.wicket.protocol.http.SecondLevelCacheSessionStore$SecondLevelCachePageMap.get(SecondLevelCacheSessionStore.java:310)
     at org.apache.wicket.Session.getPage(Session.java:779)
     at 
org.apache.wicket.request.AbstractRequestCycleProcessor.resolveRenderedPage(AbstractRequestCycleProcessor.java:458)
     at 
org.apache.wicket.protocol.http.WebRequestCycleProcessor.resolve(WebRequestCycleProcessor.java:144)

...............

the field that is null is probably because it's not loaded by Spring. Anyway, 
why should it call hashCode(), and how could I avoid it?

Best,
Martin