问题描述：使用CocoaPods加了TestFlight类库：pod 'TestFlightSDK', '~> 2.0.2'
报如下错误："_OBJC_CLASS_$_TestFlight", referenced from:
问题解决：先是觉得会不会和Crashlytics冲突，结果Google了很多，试了很多，最后试了如下方子成功！
In the Build Settings, do not override "Other Linker Flags". If it is bold, select it press backspace, it should be back to its normal state. If it is not fixed, delete all flags, remove and reinstall Pods and that should fix it.
可能是Other Linker Flags自定义的时候CocoaPods就不更新的关系。