...in particular, I do not think that an error message is a good solution for a problem. I would better disable that button (or hide it) in case Cycles is not used. What is still missing, I think, is a button that simply put the solver constraint on the camera (without anything else related to adding objects to the scene I mean).
Sep 11 2018
Sep 6 2018
I do not think it can be considered "resolved": the commit considered as the Fix, as far as I understand, works only with Cycles, and I (and I think a lot of people like me) do not understand how to switch the Motion Tracker to Cycles (switching it in the render tab does not work).