Remove usage of com.sun.istack.internal.NotNull

Jenkins found a little usage of a com.sun package in #1437 that worked fine on our local workstations - here's my quick attempt at a fix :-)

I'm not sure if this is the correct way, making the PR here then merging it immediately so we have a place to discuss it real quick if it needs a better fix. 

@Limeth & @immortius - ping!