iOS application closes when entered into foreground from background

I have a problem when my application enters foreground from background. It crashes when entering foreground. I am not able to figure out where it is crashing as it is not giving any warning in console.

I don't have an开发者_如何学JAVAy background implementation. Application should store its state when going into background and resume it when entering foreground from previous state.

What is the best way to handle multitasking in iOS?