Resolves: tdf#118862 new instances cannot be edited

Change-Id: I9b10ff4f043949c7140852ac9ad9106ca611c98c
Reviewed-on: https://gerrit.libreoffice.org/57963
Tested-by: Jenkins
Reviewed-by: Eike Rathke <erack@redhat.com>

svx/source/form/datanavi.cxx

@@ -1756,7 +1756,7 @@ namespace svxform
 
     bool DataNavigatorWindow::IsAdditionalPage(sal_uInt16 nId) const
     {
-        return m_pTabCtrl->GetPagePos(nId) >= 3;
+        return m_pTabCtrl->GetPageName(nId).isEmpty();
     }
 
     IMPL_LINK( DataNavigatorWindow, MenuActivateHdl, MenuButton *, pBtn, void )
@@ -1910,8 +1910,10 @@ namespace svxform
                 XFormsPage* pPage = GetCurrentPage( nId );
                 DBG_ASSERT( pPage, "DataNavigatorWindow::SetPageModel(): no page" );
                 if (IsAdditionalPage(nId) || m_pTabCtrl->GetPageName(nId) == "instance")
+                {
                     // instance page
                     nPagePos = m_pTabCtrl->GetPagePos( nId );
+                }
                 m_bIsNotifyDisabled = true;
                 OUString sText = pPage->SetModel( xFormsModel, nPagePos );
                 m_bIsNotifyDisabled = false;