I downloaded and tried it with the same GRD products you used and it works fine if you apply the dem assisted coregistration first and then terrain correct. I also tried terrain correcting first and then running the dem assisted coregistration and this resulted in empty output. I think the dem assisted coreg is not expecting map projected data. if you want to terrain correct first then you could apply create stack however, I believe orbit correction, dem assisted coreg and finally terrain correction should give the best result.