Caused by: java.lang.IllegalStateException: Attempting to execute an operation on a closed EntityManager.
	at org.eclipse.persistence.internal.jpa.EntityManagerImpl.verifyOpen(EntityManagerImpl.java:1665)
	at org.eclipse.persistence.internal.jpa.EJBQueryImpl.getResultList(EJBQueryImpl.java:721)
	at ru.vac.pagecode.base.PageCodeBase.getNextDTOId(PageCodeBase.java:102)
	at ru.vac.pagecode.cl.InsCompany.saveNewCompany(InsCompany.java:37)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.sun.el.parser.AstValue.invoke(AstValue.java:234)
	at com.sun.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:297)
	at javax.faces.event.MethodExpressionActionListener.processAction(MethodExpressionActionListener.java:153)
	... 36 more

package ru.vac.ejb;

import java.io.Serializable;
import java.util.List;

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;

import ru.vac.jpa.common.CommonDTO;

@Stateless(mappedName = "ejb/PolisCommonEJB")
@SuppressWarnings("unchecked")
public class CommonEJB implements CommonEJBLocal {
	@PersistenceContext(unitName="VACJPA")
	private EntityManager em;
	
    public CommonEJB() { }

	public <T> List<T> findAll(Class<T> c) {
    	return (List<T>) em.createQuery("SELECT t FROM " + c.getSimpleName() + " t").getResultList();
    }

...итд

public int getNextDTOId(CommonEJBLocal common, Class<? extends CommonDTO> c) {
	Query query = common.createQuery("SELECT MAX(v.id) FROM " + c.getSimpleName() + " v");
	List result = query.getResultList();
		
	if (result.size() > 0)
		return (Integer) result.get(0) + 1;
	
	return -1;
}

public class InsCompany extends PageCodeBase {
	private static final long serialVersionUID = 1L;
	
	@EJB
	private CommonEJBLocal common;

	public void saveNewCompany() {
		System.out.println(getNextDTOId(common, Inscompany.class));
	}
}