Deleting a ticket that has already been deleted crashes the server.

[Brojowski]

calling DELETE /tickets/ID after the ticket has been removed crashes the server with the exception

    TypeError: Cannot read property '_id' of null
        at /var/www/kenthackenough/app/tickets/controller.js:78:22
        at Query.<anonymous> (/var/www/kenthackenough/node_modules/mongoose/lib/query.js:2120:28)
        at /var/www/kenthackenough/node_modules/mongoose/node_modules/kareem/index.js:177:19
        at /var/www/kenthackenough/node_modules/mongoose/node_modules/kareem/index.js:109:16
        at doNTCallback0 (node.js:407:9)
        at process._tickCallback (node.js:336:13)