help me with this JSF form......

Hi guys,
i'm a new user of JSF and i've a big question for you.
I've a login page in which the user has to compile three input fields,
a name ,a password and a research team.
The research team is a drop-down list that comes from a query on a
mysql db.
I've this error

javax.servlet.ServletException: javax.servlet.jsp.JspException: Value
binding '#{authenticationBean.teams}'of UISelectItems with
component-path {Component-Path : [Class:
javax.faces.component.UIViewRoot,ViewId: /Login.jsp][Class:
javax.faces.component.html.HtmlForm,Id: _id1][Class:
javax.faces.component.html.HtmlPanelGrid,Id: _id2][Class:
javax.faces.component.html.HtmlPanelGrid,Id: _id4][Class:
javax.faces.component.html.HtmlSelectOneMenu,Id: _id11][Class:
javax.faces.component.UISelectItems,Id: _id12]} does not reference an
Object of type SelectItem, SelectItem[], Collection or Map but of type
: null

It's two weeks i try to solve it, but i don't understand what is the
problem.
In my Login.jsp i've done

<h:selectOneMenu value="#{authenticationBean.teamName}">
    <f:selectItems value="#{authenticationBean.teams}" />
</h:selectOneMenu>

and in my authenticationBean i've

private ArrayList<SelectItem> teams;
public String getTeamName() {
        return teamName;
    }

    public void setTeamName(String teamName) {
        this.teamName = teamName;
    }
public ArrayList<SelectItem> getTeams() throws SQLException
{//teamList()
        try{
            DataSource dataSource=Singleton.getInstance().getDataSource();
            Connection conn=dataSource.getConnection();
            if (conn != null) {
                Statement stmt = conn.createStatement();
                ResultSet rst = stmt.executeQuery("select * from researchteam");
                ArrayList<SelectItem> teams = new ArrayList<SelectItem>();
                while (rst.next())
                    teams.add(new
SelectItem(rst.getString("teamname"),rst.getString("teamname")));
                rst.close();
                conn.close();

            }
        }
        catch(Exception e){};

        return teams;
    }

i'm not able to solve my problem,
can someone help me?
Please, help me i'm a new user of Jsf..