Add checks in some places to see if a response doesn't exist

Error that pops up to when you have a callback to an FB.ui method to check for a response, as suggested in the docs: https://developers.facebook.com/docs/sharing/reference/share-dialog

If a user clicks 'cancel' or exits out of a share dialog, the response seems to be undefined. there's an error that says "cannot read property 'error' of undefined."

This seems to fix it.