If my opponent has no benched Pokemon, does the current Active Pokemon become poisoned if I attack with Roserade's "Inviting Poison"?
No, it does not become poisoned. If there's no switch to the bench there's no "new Active Pokemon" to become poisoned.
Source: Ultra Prism FAQ (2018-01-19), TPCi Rules Team (2018-02-01)

Relevant cards

Note: All applicable cards may not be listed below.
SM5_5