[Struts] Null Pointer Exception

Bonjour,
dans mon appli web développée avec Struts, il arrive qu'elle se plante et de temps en temps, sans savoir pourquoi j'ai l'erreur suivante :
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description The server encountered an internal error () that prevented it from fulfilling this request.
 
exception 
 
javax.servlet.ServletException
	org.apache.struts.action.RequestProcessor.processException(RequestProcessor.java:523)
	org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:421)
	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:743)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
	sun.reflect.GeneratedMethodAccessor181.invoke(Unknown Source)
	sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	java.lang.reflect.Method.invoke(Method.java:585)
	org.apache.catalina.security.SecurityUtil$1.run(SecurityUtil.java:243)
	java.security.AccessController.doPrivileged(Native Method)
	javax.security.auth.Subject.doAsPrivileged(Subject.java:517)
	org.apache.catalina.security.SecurityUtil.execute(SecurityUtil.java:275)
	org.apache.catalina.security.SecurityUtil.doAsPrivilege(SecurityUtil.java:161)
 
 
root cause 
 
java.lang.NullPointerException
	utils.LogIn.execute(Unknown Source)
	org.apache.struts.action.RequestProcessor.processActionPerform(RequestProcessor.java:419)
	org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:224)
	org.apache.struts.action.ActionServlet.process(ActionServlet.java:1196)
	org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:743)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:856)
	sun.reflect.GeneratedMethodAccessor181.invoke(Unknown Source)
	sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	java.lang.reflect.Method.invoke(Method.java:585)
	org.apache.catalina.security.SecurityUtil$1.run(SecurityUtil.java:243)
	java.security.AccessController.doPrivileged(Native Method)
	javax.security.auth.Subject.doAsPrivileged(Subject.java:517)
	org.apache.catalina.security.SecurityUtil.execute(SecurityUtil.java:275)
	org.apache.catalina.security.SecurityUtil.doAsPrivilege(SecurityUtil.java:161)
Pouvez-vous me guider svp :(