Oh my! That's an interesting thing to encounter... That section of the game is a work in progress and shouldn't be available through normal means. Just curious, how did you get to the garden?
The Witch Hut and the Garden are both accessible for me on the latest Android release. I get the same [exception] when starting from Slo-Grow, but selecting [Ignore] on the bottom left allowed me to continue to the game.
I recently downloaded the Steam demo, and can confirm that the [Wishlist Mushroom Musume on Steam today!] banner is in the same place on PC that the Witch Hut is on mobile.