NoMethodError: [!] undefined method `real_path' for nil:NilClass

Hi xcodeproj contributors,

Some days ago my `fastlane` script failed. So I initially opened an [issue](https://github.com/fastlane/fastlane/issues/13667) on the `fastlane` repository.

But it ended up being an issue related to `xcodeproj`. So I am forwarding the issue here.

[NoMethodError: [!] undefined method `real_path' for nil:NilClass #13667](https://github.com/fastlane/fastlane/issues/13667)

TL;DR I had a dead reference to a library into my `project.xcodeproj`.

I think the best way to understand is still to read the whole thread.
The good news is that we finally figured out why `xcodeproj` crashes, and it crashes for a good reason.

But maybe we could make the reason of this crash more obvious in case it happens to others ?

Start by reading the linked fastlane issue #13667 and the report's description of a dead library reference in project.xcodeproj. Reproduce the NoMethodError if possible, then determine what clearer failure reason is needed when xcodeproj encounters that reference.

Written by the indexing model from the issue text.