1) Stop the project and run Product-> Clean (or shortcut <Shift>+ <Command>+ <K>)
2) Right-click the project name in the root of the groups and files tree on the left side of your project window and select "Open in Finder". In the Finder window, locate the Build folder and delete it.
3) If still not working, search for your xib in the Goups tree and in each branch from your environment (Finder-> File-> Find ...).
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