When adding or removing a pubspec.yaml while the widget previewer is running, it's possible to encounter the following crash:

Thread 0 main threadStateError: Bad state: No element
at ListBase.firstWhere(list.dart:132)
at PreviewManifest._calculatePubspecHashes(preview_manifest.dart:69)
at PreviewManifest.updatePubspecHash(preview_manifest.dart:146)
at PreviewPubspecBuilder.populatePreviewPubspec(preview_pubspec_builder.dart:174)
at <asynchronous gap>(async)

An example of this situation is a user creating a new workspace project in the previewed directory. The preview detection logic sees that a pubspec.yaml has been updated and attempts to update its hash code in the preview manifest. This is done by trying to find the first FlutterProject which has a pubspec at the location of the new pubspec.yaml. Since this project is new, the previewer isn't aware of it and the firstWhere(...) call throws since there's no match.