Xcode升级后，打开项目出现了
Multiple commands produce '/Users/lazyduan/Library/Developer/Xcode/DerivedData/BWC-cnppauzubkvhgxehjlcszowsqgbr/Build/Products/Debug-iphoneos/BWC.app':的错误。
处理方法：
Go to Xcode->File->Workspace/Project Settings-> Build System -> Legacy Build System!
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