What does the line "the sun should never be set upon an argument" mean?

Read the excerpt from act 2 of a doll's house. nora: [quickly] he mustn't get the letter. tear it up. i will find some means of getting money. krogstad: excuse me, mrs. helmer, but i think i told you just now— nora: i am not speaking of what i owe you. tell me what sum you are asking my husband for, and i will get the money. krogstad: i am not asking your husband for a penny. nora: what do you want, then? krogstad: i will tell you. i want to rehabilitate myself, mrs. helmer; i want to get on; and in that your husband must me. for the last year and a half i have not had a hand in anything dishonourable, amid all that time i have been struggling in most restricted circumstances. i was content to work my way up step by step. now i am turned out, and i am not going to be satisfied with merely being taken into favour again. i want to get on, i tell you. i want to get into the bank again, in a higher position. your husband must make a place for me— nora: that he will never do! krogstad: he will; i know him; he dare not protest. and as soon as i am in there again with him, then you will see! within a year i shall be the manager's right hand. it will be nils krogstad and not torvald helmer who manages the bank. nora: that's a thing you will never see! krogstad: do you mean that you will—? nora: i have courage enough for it now. krogstad: oh, you can't frighten me. a fine, spoilt lady like you— nora: you will see, you will see. krogstad: under the ice, perhaps? down into the cold, coal-black water? and then, in the spring, to float up to the surface, all horrible and unrecognisable, with your hair fallen out— nora: you can't frighten me. krogstad: nor you me. people don't do such things, mrs. helmer. besides, what use would it be? i should have him completely in my power all the same. nora: afterwards? when i am no longer— krogstad: have you forgotten that it is i who have the keeping of your reputation? [nora stands speechlessly looking at him.] well, now, i have warned you. do not do anything foolish. when helmer has had my letter, i shall expect a message from him. and be sure you remember that it is your husband himself who has forced me into such ways as this again. i will never forgive him for that. goodbye, mrs. helmer. [exit through the hall.] what conflict does krogstad introduce? krogstad tells nora that he has written a letter telling helmer about her affair with the doctor. krogstad refuses to forgive helmer unless nora finds a way to come up with more money. krogstad tries to blackmail nora into getting helmer to keep him at the bank by exposing her forgery. krogstad plans to take helmer’s job managing the bank and ruin nora’s reputation while doing so.