Newtonsoft Json helper class
Your class name is Json yet your methods contain this acronym in all-uppercase: JSON. The naming conventions say: "Do capitalize only the first character of acronyms with three or more characters, except the first word of a camel-cased identifier."



fs isn't a clear variable name. Same for sw (why not use streamWriter?) and sr (streamReader). I'd even include writer.



Considering that you don't use var serializer = new JsonSerializer(); until later, why not delay this call until you need it?



I'm bothered by catch (Exception). Are you certain you don't want to know that something went wrong, and what exactly went wrong?