Unless a card says, or it doesn't make sense otherwise, abilities are only active when the card is on the battlefield. Everywhere else, they don't do anything.
This is the case with hexproof, it is only relevant when the card is on the battlefield. While the card is on the stack (ie: just after you've cast it and before it has resolved), it doesn't have any effect.
Dissolve
Yes, Hexproof only functions when Witchstalker is on the battlefield. While it is on the stack Dissolve can counter it.
dissolve
Yes, your opponent can counter your witchstalker.
Unless a card says, or it doesn't make sense otherwise, abilities are only active when the card is on the battlefield. Everywhere else, they don't do anything.
This is the case with hexproof, it is only relevant when the card is on the battlefield. While the card is on the stack (ie: just after you've cast it and before it has resolved), it doesn't have any effect.