Pod Error on File Path

[HuXu77]

Running flutter build ios in example director results in this.

[!] Invalid `Podfile` file: no implicit conversion of nil into String.

     #  from /Users/mitchellclay/flutter_kakao_map/example/ios/Podfile:54
     #  -------------------------------------------
     #      unless File.exist?(copied_framework_path)
     >        FileUtils.cp_r(File.join(cached_framework_dir, 'Flutter.framework'), copied_flutter_dir)
     #      end
     #  -------------------------------------------
     

[HuXu77]

This is returning nil generated_xcode_build_settings['FLUTTER_FRAMEWORK_DIR'];