Xporter should validate Permission Schemes when user perform the export

Xporter isn't able to validate correctly the permission schemes when the user performs an export on backlog context.

Steps to reproduce the error:

• Firstly, create a new permission scheme with two types: "Task" and "Story"
• Now, go to the Backlog page to perform an export from the backlog context when you have a issue type "Bug"
• The user won't be able to perform the export.