As suggested, the error message is telling you that you're trying to access a member of an object that doesn;t exist. You need to debug your code. You ALWAYS need to debug your code first. Place a breakpoint on that and, when execution breaks, use the debugger to see whether Driver or Driver.Content is null and then work backwards to determine why and what you need to do to fix it.
Why would it? Look at your method. It receives a Button via a parameter and then proceeds to use the Content of that in code. How is the compiler supposed to know whether you will have set the Content of that Button at run time when you call that method?