Your app.config needs to be set up in a very specific way for that to work.
If it isn't set up that way then ConnectionStrings will be null.
I would suggest that you do the following
1. Create a project/app that ONLY gets the connection string
2. Create a app.config that ONLY has that connection string.
3. Verify in that app that ConnectionStrings is not null before using it.
4. Get the connection string property and print it.
Then if that doesn't work then you can post the entire class and the entire app.config. Do not include real info in the app.config all you care about is that it get a value not what is in it.