This. If you're pulling over a reckless driver generated by PoliceHelper, then LCPDFR will not recognise it as a pursuit and therefore the pursuit specific backup keys (F10, F11, etc) will not work. You get the error in .PullOver as the PoliceHelper script is making the vehicle evade you, while LCPDFR is trying to make it pull over, as they aren't linked and LCPDFR treats it as a normal vehicle, and therefore they clash, and PoliceHelper wins. This is my theory, it's happened to me a few times. When in a PoliceHelper pursuit I normally just handle it by myself.
Hope this isn't too difficult to understand, it was difficult to explain