I am trying to debug my program. The breakpoint works, but when I click the Step Forward button (or do something else with the debugger), Xcode says:

I tried to remove the derived data and clean up the project, and this does not help. I am using OS X 10.11 and Xcode 7.2.
Edit: something seems to be related to my project. The code is in https://github.com/vindo-app/vindo , I am working on the menu branch. The breakpoint is in /Start/StartMenu.m models in -addItemAtURL:

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