New Post: Getting Null reference error when using Rasterizer

I guess that _lastInstalledVersion is null and that is because Ghostscript.net doesn't find the installed Ghostscript application.
Make sure that you run your application in 64 bit mode (since you installed the 64 bit version of ghostscript).
You can check if that's the case by checking the Envionment.Is64BitProcess property.

Console.WriteLine("Environment.Is64BitProcess:"+Environment.Is64BitProcess);