Have you tried either
Dim objMxDoc As IMxDocument
objMxDoc = New MxDocument
or
Dim objMxDoc As New MxDocument
I tried, but the problem persistsPrivate myApps As ESRI.ArcGIS.Framework.IApplication
in the method that starts ArcMapESRI.ArcGIS.RuntimeManager.Bind(ESRI.ArcGIS.ProductCode.Desktop)
... Dim objMxDoc As IMxDocument ... objMxDoc = New MxDocument