If you're stuck on the "Imagination Land" quest because there are no Night Thorns to remove, you can try this series of steps to fix the issue:
Untrack the quest. Go into your quest menu and select "Imagination Land" to unpin it from your objectives.
Fully restart the game. Make sure to completely close the application and then relaunch it from your console or PC launcher.
Retrack the quest. Once the game is back up, go back to your quest menu and re-select "Imagination Land" to track it again.
Level up Sadness and Joy. After performing the above steps, work on leveling up Sadness and Joy to level 7.
- Wait for new Night Thorns to spawn naturally and remove them. It might however take a few days as the Night Thorns spawn each day in one random Biome
This process should resolve the issue and allow the Night Thorns to reappear, letting you progress in the quest.