How can we have free will without sin in heaven, but can't do so on earth?
The general explanation I've heard (if I can try to summarize it) for "the problem of evil", of why a good God allows evil things to happen in the world, is that God doesn't want "robots" as he wants people to choose to worship and follow him. And if we're given that free will, then inevitably we will at some point choose not to worship God. And that's what's behind the fall and sin and generally the source of evil, that at times people won't be choosing to follow God.
Now, that all actually makes perfect sense to me. My question is: Why doesn't that same logic apply to us in heaven? (Or the New Jerusalem or whatever name you want to use to describe believers' eternity together with God.)
That is, it seems that the logical conclusion is either (1) we don't actually have free will in heaven, so we can't choose to disobey God, or (2) it is possible (somehow) for God to make a place where we both have free will and yet cannot sin (as hard as it is for us to understand). If #1 is the answer, it seems weird that God would not want "robots" on earth but would be okay with them in heaven. But if #2 is the answer, you're just left with the original problem of evil: Why, if God can make such a place, would he have made Earth in a different way that allowed evil to exist rather than in a way that both allows for free will but without sin?
My background is Baptist (non-Calvinism Protestant), but I'm interested in hearing answers across the breadth of Christian traditions.