iOS App crashes on WLAN disconnect

I am facing a strange problem with my iOS app. It is written in Objective-C and i use iOS 8.3 on my testdevice (iPhone 4s). The app uses push notifications.

When I lock my iPhone, I sometimes get the following issue: Before unlocking the device again, the wlan status symbol in the status bar disappears and when I finally unlock it, the app freezes and I have to restart it (-> The WLAN sign returns and the app works without any problems)