If you create a ring world or sphere world in your system then try and turn an asteroid into a planet it says there is no star present. If you then attempt to create a star it says that you can't create a star when there is one with a construction around it already preset. Seems to me that something here needs to give. I actually remember this from an older Version of the game but then you could still just create a new star and then make the planet... and then another ring/sphere world.
So seems this was corrected but the planet creation wasn't updated to recognize it.