If every Force-sensitive user needs a Force-sensitive master...
...who taught the first ever Force-sensitive master how to use the Force?
Logically speaking, at least one Force-sensitive individual has to have had the ability to use the Force without training or else no one would have been able to use the Force in the first place.
The Force is intuitive first; technical later. Think back to Luke's initial training with Obi-Wan and the Marksman-H combat remote droid. Kenobi didn't physically lead Luke through the "motions" of using the Force in order to blindly deflect the blaster bullets. He didn't hold Luke's hands and say, "Swing like this". He provided simple verbal commands to guide Luke into connecting with his abilities- it was up to Luke to discover what the Force actually felt like when tapped into and thus how to manipulate it. As far as I'm concerned, Luke is an untrained Force user who does end up deflecting shots from the remote droid through use of the Force- just with a little encouragement from Obi-Wan.
Rey had more "training" in the art of the Force Mind Trick than Luke did when it came to Force Precognition. Rey at least had the misfortune (but advantage) of seeing and experiencing Kylo Ren's Mind Probe which she intuitively was able to block by tapping into that specific area of the Force. From there, she need only apply her newfound power at getting into the mind of someone else.
On the other hand, Luke was told to strap on a helmet, become blind, pick up a very complicated weapon, and then deflect small blaster bullets with the "edge" of said complicated weapon- all without having the luxury of watching someone else do it or feel the ripple in the Force as another person performs the feat.
But then again, that's part of what makes Luke truly such a naturally powerful Force user.