Fixed undefined id error when no items are available for relationship field

[rinodrummer]

Fixes the first console error described by me in #3062:

create:572 Uncaught TypeError: Cannot read property 'id' of undefined
    at Object.success (create:572)
    at l (bundle.js?v=4.1.15@6c751de946a9c8511dd32eb7bfa3ca6a568849f5:2)
    at Object.fireWith [as resolveWith] (bundle.js?v=4.1.15@6c751de946a9c8511dd32eb7bfa3ca6a568849f5:2)
    at T (bundle.js?v=4.1.15@6c751de946a9c8511dd32eb7bfa3ca6a568849f5:2)
    at XMLHttpRequest.<anonymous> (bundle.js?v=4.1.15@6c751de946a9c8511dd32eb7bfa3ca6a568849f5:2)

[tabacitu]

Whoop-whoop! Thanks for the PR, I'll let @pxpm review this since fetch_or_create is his baby.

Cheers!

[tabacitu]

Sorry - this got closed automatically because I merged #3064 . Please let's create another PR with this if that alone doesn't fix both problems. @pxpm please let @rinodrummer know if that's the case.