I received a question through my blog that asked for help with an NUnitForms exception. I'm sorry to say that I no longer recommend NUnitForms as a test tool. I was using it 3 years ago but there has not been much activity in that community since then. I and some other developers joined the NUnitForms community and offered to fix some bugs we found in the May 2006 releases. We got no response from the project owner. Since then, I have not even had any email on the community's email list.
So, I had to stop using NUnitForms as a test tool.Fortunately, there is an alternative!
In February 2008, Vivek Singh released the first version of White an open source UI test tool that works with WinForms, WPF, and anything else that .Net's Windows.Automation API's can find. I've used it to test WinForms, WPF, and web applications. It should work with just about any sort of UI that runs on a Windows operating system.
It's free and it works with NUnit. You can download the latest release of White from CodePlex at this link http://white.codeplex.com/Release/ProjectReleases.aspx?ReleaseId=14789
Like any piece of software, it will have bugs in it. But, the project owner is active and there is an active community using and contributing to the project. All in all, I think you would have much better luck switching tp White than continuing to use NUnitForms.