The ViewStateUserKey property needs to be set during Page_Init.

Stack trace:
at System.Web.UI.Page.set_ViewStateUserKey(String value)
at WOT.PortalControls.WOTAbstractWebPartEx.Page_Init(Object sender, EventArgs e) in D:\Git\TOW_0715\Source\EdenredFi.WOTWeb\App_Codes\WOTAbstractWebPart.cs:line 711
at System.Web.UI.Control.OnInit(EventArgs e)
at CMS.Base.Web.UI.AbstractUserControl.OnInit(EventArgs e)
at CMS.PortalEngine.Web.UI.CMSAbstractWebPart.OnInit(EventArgs e)
at System.Web.UI.Control.InitRecursive(Control namingContainer)
at System.Web.UI.Control.AddedControl(Control control, Int32 index)
at CMS.PortalEngine.Web.UI.CMSWebPartZone.LoadWebPart(Control container, CMSWebPartZone zone, WebPartInstance part, Boolean reloadData, Boolean isVariant)

// System.Web.UI.Page
/// <summary>Assigns an identifier to an individual user in the view-state variable associated with the current page.</summary>
/// <returns>The identifier for the individual user.</returns>
/// <exception cref="T:System.Web.HttpException">The <see cref="P:System.Web.UI.Page.ViewStateUserKey" /> property was accessed too late during page processing. </exception>
// Token: 0x1700093A RID: 2362
// (get) Token: 0x060020DC RID: 8412 RVA: 0x0006966A File Offset: 0x0006786A
// (set) Token: 0x060020DD RID: 8413 RVA: 0x00069672 File Offset: 0x00067872
[Browsable(false)]
public string ViewStateUserKey
{
    get
    {
        return this._viewStateUserKey;
    }
    set
    {
        if (base.ControlState >= ControlState.Initialized)
        {
            throw new HttpException(SR.GetString("Too_late_for_ViewStateUserKey"));
        }
        this._viewStateUserKey = value;
    }
}