"Cannot place windows in MDI since no MDI host (TabbedMdiHost, StandardMdiHost, etc.) was located"

v20.1.1

Hello, A customer is seeing this issue and we're not able to reproduce it. Would you be able to give some guidance about what we might be doing wrong or how this error could occur? 

System.InvalidOperationException: Cannot place windows in MDI since no MDI host (TabbedMdiHost, StandardMdiHost, etc.) was located as the direct child of the DockSite's Workspace.
   at ActiproSoftware.Internal.ap.yDT(DockingWindow  , DockHost  , DockingWindowState  , Nullable`1  )
   at ActiproSoftware.Internal.ap.KDo(DockingWindow  , IDockTarget  , Nullable`1  , Nullable`1  , Int32  )
   at ActiproSoftware.Internal.ap.Activate(DockingWindow window, Boolean focus)
   at ActiproSoftware.Windows.Controls.Docking.DockingWindow.Activate(Boolean focus)
   at ES.Ds.GUI.Production.Scripting.ScriptEditorWnd.AddTab(ScriptEditorDocument document)

            <docking:DockSite Name="m_scriptEditorDockSite" Padding="0" Background="{StaticResource paneBackgroundBrush}" CanDocumentWindowsFloat="False" CanDocumentWindowsDragToLinkedDockSites="False" KeyDown="OnKeyDown">
                <docking:Workspace>
                    <docking:TabbedMdiHost AllowDrop="False">
                        <docking:TabbedMdiContainer>
                        </docking:TabbedMdiContainer>
                    </docking:TabbedMdiHost>
                </docking:Workspace>
            </docking:DockSite>

        private void AddTab(ScriptEditorDocument document)
        {
            if (m_scriptEditorDockSite.IsVisible)
            {
                document.Activate();
            } else
            {
                document.Open();
            }
        }