Portrait Dialogue: Missing tracking images in emails result in Object reference not set to an instance of an object exception



After upgrading to Portrait Dialogue 6.1 there have been observed issues with email tracking (note the link tracking works fine).

The link to the tracking image gives the following error:

NullReferenceException: Object reference not set to an instance of an object.]
MHWebUtilities.ET.EmailTrackerHttpHandler.ProcessRequest(HttpContext context) in d:\Build\60\Sources\DotNet\Old\WebForms\WebUtilities\ET\EmailTrackerHttpHandler.cs:82
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +341
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +69


The tracking image for «#EmailTrackImage()» function, which returns a string containing an HTML image tag, is missing from the Resources/ETImages folder for the MHWU web application (from which it is downloaded and used for tracking).  This is most likely due to use of a custom image that was wiped out during the upgrade process. 

User-added image

The default image (a transparent 1 x 3 pixels gif image) can be exchanged with other images by adding them to the Resources/ETImages folder.
A new image is used instead of the default image by replacing the Image downloaded part of the image URL.

User-added image


UPDATED: May 9, 2019
If the tracking image is missing from MHWU web application in the Resources/ETImages folder it has to be retrieved or redefined anew. The Resources/ETImages folder for MHWU web application can easily be browsed in the Internet Information Services (IIS) Manager.