“No disaster.” Proverbs 12:21 is one of the “ideal” promises in the Word of God. (See commentary on Prov. 19:5).
“Will come upon.” The Hebrew verb is anah (#0579 אָנַה), and in the qal aspect of the Hebrew verb it means “to meet or encounter.” However, in this verse, it is in the pual aspect of the verb, the causal aspect, and it means “to be sent” or “to be allowed to meet” (Brown-Driver-Briggs Hebrew and English Lexicon; Theological Wordbook of the Old Testament).
The point of the proverb is not that the righteous are immune to “disaster.” Proverbs has many verses showing the righteous can suffer (Prov. 18:5; 24:15; 28:10). And other books like Job show it very clearly as well. Job was a righteous man who lost his children, his wealth, his health, and the respect of his friends. The point of this verse is that God is always good, and He never sends disaster upon the righteous.
The point does need to be made, however, that in the general context of Proverbs, righteous people do well and wicked people do badly, and there is certainly an overtone of that in this verse.