GameStateManagement Sample problem.

Sorry, but you are unlikely to get it.
At least until you sort out that question.
Just dumping your entire file here is like saying "I can't be bothered - you sort it out" and is not likely to get many people to look at your problem. Would you want to wade through that much poorly comments code to try and work out what line your error is related to?

Edit your question, remove the code dump, and paste only the relevant code fragment. Somewhere between line 75 and 100 should be a start...
Use the "Improve question" widget to edit your question and provide better information.

As Wes said - we didn't say we "couldn't help" - just that we won't while you just dump the code on us and walk away.
Remember that nobody here gets paid to help, and that you don't get charged for help. The easier you make it for us to help you, the more help you are likely to get! :laugh:

"Could do with some help".. Sure first thing is to chop this code dump down to some sort of reasonable size. Dumping all of your code on here to see if someone will debug it for you is rude.
Second thing is read the error. It tells you what the problem is and even tells you which line of code the error occurs on. Go to that line and fix whatever the error tells you.