问在初始化的时候，pod install之后，提示如下错误，怎么解决？

MacBook-Pro ios % pod install

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

# from /Users/admin/Desktop/face-app/ios/Podfile:54

# -------------------------------------------

# unless File.exist?(copied_framework_path)

> FileUtils.cp_r(File.join(cached_framework_dir, 'Flutter.framework'), copied_flutter_dir)

# end

# -------------------------------------------