I think you are confusing the independent field for the Navier Stokes equations set with the independent field for the advection equations set, the fields do not depend on each other (even when coupled). The independent field for the advection problem specifies the advection field. For you coupled problem this will be your Navier Stokes dependent field. For the auto-created field it should be one variable with number of components given by the number of dimensions for the problem.