XmlPokeNS does not work as expected

I am trying to replace the version number within a clickonce app.manifest. I wrote a test for it and fixed the NullReference bug in XmlPokeNS, but the test is still red. Did this function work before?

The MSBuildReleaseExt function is a former extension of mine and can safely be removed. I don't know how to remove this difference from the pull request. Sorry.