While in Fred.vb, Page.vb, Ted.vb etc you can use this syntax:

Dim shellTabItem As WinUI.Controls.ShellTabItem = MainAddIn.FindShellTabItem("Ted4Articles", Main)

in Article.vb you have to call it differently:

Dim tab As WinUI.Controls.ShellTabItem = MainAddIn.FindShellTabItem(TabItemName, TxtDesigner.Main)

otherwise, you will get the error:

Option Strict On disallows implicit conversions from 'IApp' to 'TeraDP.GN4.WinUI.Contracts.IMain'.