It's in the version 0.9 andriod I started a new game several times Idk if I didn't hit the button but I think I clicked it over 10 times.
1. Make sure you have enough coins to buy potion "1000 coins"
2. While going back first click on back button in shop menu
3. Choose 2nd option to exit shop. (You can hold touch and drag down on options to ensure 2nd option is highlighted before lifting your finger)
Here is a video for this "I can't buy here since I have 0 coins"