Crash when return

[Jinkeycode]

I add LiquidLoader when ViewDidLoad, but the app will crash when I return to the previous view。

0x101e02588 <+44>: brk #0x1

the breakpoint is：
let frame = self.loader!.frame.center.minus(self.loader!.frame.origin)

fatal error: unexpectedly found nil while unwrapping an Optional value

[devilon]

me too, do you resolve it?