If Generated.xcconfig has not been #included in the Xcode project configuration, FLUTTER_ROOT is undefined and Xcode will be unable to locate xcode_backend.sh. In the flutter create project template, we should include something similar to the following boilerplate above the execution of the xcode_backend.sh script:

if [ ! -f "$FLUTTER_ROOT"/packages/flutter_tools/bin/xcode_backend.sh" ]; then
  echo "xcode_backend.sh not found"
  echo "FLUTTER_ROOT is '$FLUTTER_ROOT'"
  echo "Verify that your Xcode project is configured to use the Debug or Release profile"
  exit 1
fi