I have noticed a few reviews with one star and no comments. The student asked me a question on Q and A section regarding the progress of assignments. He has completed assignments but the progress is not visible on his dashboard. I told him to connect over mail - firstname.lastname@example.org. The support team is so busy handling other queries but the student was in a hurry to leave a review. I flagged his review and I am confident enough that Udemy won't remove his one star.
At times, it is very disturbing to see such reviews due to interface problems. It's a sincere request to the Udemy team to kindly rectify the problems on the dashboard so that students may not leave a review like this and destroy the course. In addition, there are chances that student leave the Instructor and ask for a refund that is a loss to Instructor and Udemy also.
First time 1-star reviews?! I wouldn't worry if I were you because there is literally nothing you can do about it. Rating is purely under students' power and they can do whatever they want with it. And a lot of students are abusing it, unfortunately. They think if they don't like even the slightest thing they are obliged to give it the lowest rating ever.
As an example (and no this is not a joke) student gave me the low rating because I snapped fingers in one of my lectures (he called it "unprofessional" and bashed the course with the low review).
Now about the effect on sales. Coming from my 1500+ reviews, I can safely say that courses jump up and down with ratings wildly especially at the beginning (you have only 78 reviews in total, so I guess it is just a beginning for the course). As an example, one of my courses went as low as 2.9 on ratings because of such students but then it went back up now sitting at 4.1. And at any time between those, I didn't see a drop in sales, only steady growth.
As for the refunds. Yes, when a student is refunded, both Udemy and instructor are losing what they earned from the course, there is nothing you and Udemy can do about that, It is the policy. And yes, a lot of students are abusing it, by going through the course, pretending they didn't like it (leaving 1-star review with no comment) and refunding it. Yes, such people exist and they are abundant. But again, you can not do anything about it. So please, do not worry.
Just focus on improving your courses, on new courses and everything will be fine.