In TMS Web Core V2.6 beta, method WEBLib.Controls.TControl.UpdateElementSize we have an issue with the line 5434:
if Assigned(ctrl.Parent) and (ctrl.Parent.ElementHandle.style.getPropertyValue('position') = 'absolute') then
It should be fixed to
if Assigned(ctrl.Parent) and Assigned(ctrl.Parent.ElementHandle) and (ctrl.Parent.ElementHandle.style.getPropertyValue('position') = 'absolute') then
The issue occurs, if there is a TTMSFNCWXHTMLMemo within a TWebPanel on a modal form.