Solomon quotes the promise to David with an "if" (6:16). Did the promise depend on David's sons?

The chapter doesn't say how the if fits the promise, and this answer doesn't guess. Solomon first thanks the LORD, "who have kept with your servant David my father that which you promised him" (6:15). Then he asks him to keep "that which you have promised him" (6:16), and quotes it with an if: "There shall not fail you a man in my sight to sit on the throne of Israel, if only your children take heed to their way, to walk in my law as you have walked before me" (6:16). 1 Kings ends the saying differently: "to walk before me as you have walked before me" (1 Kings 8:25). In 1 Chronicles the LORD's word through Nathan had no if: "I will settle him in my house and in my kingdom forever. His throne will be established forever" (1 Chronicles 17:14). David later gave the LORD's words about Solomon with one: "I will establish his kingdom forever if he continues to do my commandments and my ordinances, as it is today" (1 Chronicles 28:7). In the next chapter the LORD answers Solomon with an if too (7:17-18), and later the book says, "However the LORD would not destroy David’s house, because of the covenant that he had made with David" (21:7). These books say both, and don't set out how the forever and the if fit together, and this answer leaves them side by side.