Assertion failure in -[UIApplication _runWithMainScene:transitionContext:completion:],

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Assertion failure in -[UIApplication _runWithMainScene:transitionContext:completion:], /BuildRoot/Library/Caches/com.apple.xbs/Sources/UIKit_Sim/UIKit-3512.30.14/UIApplication.m:3315

其实就是忘记了加上

 self.window.rootViewController = nav;