Cannot animate the 'IsOpen' property on a 'MaterialDesignThemes.Wpf.PopupEx'

[nathenxbrewer]

When packaging a WPF project as a Class Library, I see the following error message when trying to show the OpenDirectoryDialog.

System.Windows.Media.Animation.AnimationException: 'Cannot animate the 'IsOpen' property on a 'MaterialDesignThemes.Wpf.PopupEx' using a 'System.Windows.Media.Animation.BooleanAnimationUsingKeyFrames'. For details see the inner exception.'

Inner Exception NullReferenceException: Object reference not set to an instance of an object.

I have no issues using MaterialDesignThemes as a Class Library as well as I do not receive the above error when running the program as a Windows Application.

The reason for using a class library is that the program is a plugin to another program I have deployed.