Pod install giving Error

[gopalkohli]

Below is the error when I m trying pod install

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

 #  from /Volumes/Develop/flutter/samples/printing/esc_pos_blue/ios/Podfile:57
 #  -------------------------------------------
 #      unless File.exist?(copied_framework_path)
 >        FileUtils.cp_r(File.join(cached_framework_dir, 'Flutter.framework'), copied_flutter_dir)
 #      end
 #  -------------------------------------------