关于xcode中在模拟器测试uitextfield按下时崩溃

-[Swift._NSContiguousString objectAtIndex:]: unrecognized selector sent to instance 0x7f95cda7bdd0
 *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Swift._NSContiguousString objectAtIndex:]: unrecognized selector sent to instance 0x7f95cda7bdd0'
 *** First throw call stack:
 (
 0 CoreFoundation 0x0000000105816f35 exceptionPreprocess + 165
 1 libobjc.A.dylib 0x0000000108596bb7 objc_exception_throw + 45
 2 CoreFoundation 0x000000010581e04d -[NSObject(NSObject) doesNotRecognizeSelector:] + 205
 3 CoreFoundation 0x000000010577627c ___forwarding_ + 988
 4 CoreFoundation 0x0000000105775e18 CF_forwarding_prep_0 + 120
 5 CoreFoundation 0x0000000105794ddc CFStringTokenizerCreate + 796
 6 UIKit 0x000000010759dd58 __91-[_UITextInputControllerTokenizer _getClosestTokenRangeForPosition:granularity:downstream:]_block_invoke44 + 253
 7 UIFoundation 0x00000001118e906b -[NSTextStorage coordinateReading:] + 42
 8 UIKit 0x000000010759da29 -[_UITextInputControllerTokenizer _getClosestTokenRangeForPosition:granularity:downstream:] + 322
 9 UIKit 0x000000010729979c -[UITextInputStringTokenizer positionFromPosition:toBoundary:inDirection:] + 72
 10 UIKit 0x0000000107299e4e -[TIDocumentState(UITextInputAdditions) _contextBeforePosition:inDocument:] + 68
 11 UIKit 0x0000000107299f91 -[TIDocumentState(UITextInputAdditions) initWithDocument:] + 110
 12 UIKit 0x0000000107299aea +[TIDocumentState(UITextInputAdditions) documentStateOfDocument:] + 50
 13 UIKit 0x0000000107030683 -[UIKeyboardImpl syncDocumentStateToInputDelegateWithExecutionContext:] + 288
 14 UIKit 0x0000000107030edb -[UIKeyboardImpl updateForChangedSelectionWithExecutionContext:] + 296
 15 UIKit 0x000000010759e914 -[UIKeyboardTaskQueue continueExecutionOnMainThread] + 332
 16 UIKit 0x000000010702b71f -[UIKeyboardImpl setDelegate:force:] + 2031
 17 UIKit 0x00000001072f9a12 -[UIPeripheralHost(UIKitInternal) _reloadInputViewsForResponder:] + 902
 18 UIKit 0x0000000106fb631d -[UIResponder becomeFirstResponder] + 468
 19 UIKit 0x0000000106eabe03 -[UIView(Hierarchy) becomeFirstResponder] + 99
 20 UIKit 0x0000000107572ad7 -[UITextField becomeFirstResponder] + 51
 21 UIKit 0x00000001071fa9c1 -[UITextInteractionAssistant(UITextInteractionAssistant_Internal) setFirstResponderIfNecessary] + 177
 22 UIKit 0x00000001071fca30 -[UITextInteractionAssistant(UITextInteractionAssistant_Internal) oneFingerTap:] + 2263
 23 UIKit 0x00000001071f22e6 _UIGestureRecognizerSendActions + 262
 24 UIKit 0x00000001071f0f89 -[UIGestureRecognizer _updateGestureWithEvent:buttonEvent:] + 532
 25 UIKit 0x00000001071f5ba6 __UIGestureRecognizerUpdate_block_invoke662 + 51
 26 UIKit 0x00000001071f5aa2 UIGestureRecognizerRemoveObjectsFromArrayAndApplyBlocks + 254
 27 UIKit 0x00000001071ebb1d _UIGestureRecognizerUpdate + 2796
 28 UIKit 0x0000000106e85ff6 -[UIWindow _sendGesturesForEvent:] + 1041
 29 UIKit 0x0000000106e86c23 -[UIWindow sendEvent:] + 667
 30 UIKit 0x0000000106e539b1 -[UIApplication sendEvent:] + 246
 31 UIKit 0x0000000106e60a7d _UIApplicationHandleEventFromQueueEvent + 17370
 32 UIKit 0x0000000106e3c103 _UIApplicationHandleEventQueue + 1961
 33 CoreFoundation 0x000000010574c551 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION_ + 17
 34 CoreFoundation 0x000000010574241d __CFRunLoopDoSources0 + 269
 35 CoreFoundation 0x0000000105741a54 __CFRunLoopRun + 868
 36 CoreFoundation 0x0000000105741486 CFRunLoopRunSpecific + 470
 37 GraphicsServices 0x000000010ade19f0 GSEventRunModal + 161
 38 UIKit 0x0000000106e3f420 UIApplicationMain + 1282
 39 AnyThingBack 0x0000000104be47de top_level_code + 78
 40 AnyThingBack 0x0000000104be481a main + 42
 41 libdyld.dylib 0x00000001092e5145 start + 1
 42 ??? 0x0000000000000001 0x0 + 1
 )
 libc++abi.dylib: terminating with uncaught exception of type NSException