当我尝试单击一个按钮时，它移动到下一个UIViewcontroller，然后从那里单击后退按钮，它返回到之前的UIViewcontroller，突然崩溃了。 以下是崩溃报告
2013-08-19 12:02:19.849 MyApp[851:907] *** Terminating app due to uncaught exception 'NSGenericException', reason: '*** Collection <CALayerArray: 0x1f1f1120> was mutated while being enumerated.' *** First throw call stack: (0x34a1d2a3 0x3c67797f 0x34a1cd85 0x36827ca3 0x36827d33 0x36827d33 0x36827d33 0x36827d33 0x36827b9b 0x36a0b585 0x368538af 0x368a3f19 0x3683aab3 0x368af8ef 0x365e4c01 0x3ca8e4b7 0x3ca8fdcb 0x349f0f3b 0x34963ebd 0x34963d49 0x384e02eb 0x36879301 0xdc439 0xdc3c0) libc++abi.dylib: terminate called throwing an exception