Terminating app due to uncaught exception ' nsunknownkeyexception ' This class isn't key value coding-compliant for the key

Terminating app due to uncaught exception ' nsunknownkeyexception ', Reason: ' [Viewcontroller > SetValue: Forundefinedkey:]: This class was not key value coding-compliant for the key backbtn. '

First case: Xib file attribute output connection error, ibout or forgot to close the input port

Second case: The introduction of third-party SDK will be error, the import bundle file nib file problems, need to re-import bundle resource file if the Common SDK in the bundle file inside the nib file needs to be placed in a different path to call, Easy to find the controller class corresponding to the NIB

Terminating app due to uncaught exception ' nsunknownkeyexception ' This class isn't key value coding-compliant for the key