It always close unexpectedly even today, Opening the app works fine but as I play for around 2 minutes+ (chatting with the npcs, Exploring the map, ...) It starts closing on its own. I'm using RealmeC11 for this and other games just works fine.
I looked at the specifications of your device and I have a hunch that your phone has a weak CPU and not enough RAM. Does your device have 1-1.5 GB of available RAM?