change_notifier_provider compile error

[PineyZhang]

I'm trying to run the change_notifier_provider example and am getting the error below when trying to run on an iPhone 12 Pro simulator. I'm running in VS Code.

$ flutter run
Multiple devices found:
M2002J9E (mobile) • 192.168.0.151:5555 • android-arm64 • Android 11 (API 30)
iPhone 12 Pro (mobile) • 13624E90-6880-4713-A2CE-03E4749BB26C • ios • com.apple.CoreSimulator.SimRuntime.iOS-14-5 (simulator)
Chrome (web) • chrome • web-javascript • Google Chrome 91.0.4472.77
[1]: M2002J9E (192.168.200.151:5555)
[2]: iPhone 12 Pro (13624E90-6880-4713-A2CE-03E4749BB26C)
[3]: Chrome (chrome)
Please choose one (To quit, press "q/Q"): 2
Launching lib/main.dart on iPhone 12 Pro in debug mode...
Running pod install... 2,090ms
CocoaPods' output:
↳

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

 #  from /Users/zzz/Documents/workspace/flutter/flutter_architecture_samples/change_notifier_provider/ios/Podfile:57
 #  -------------------------------------------
 #      unless File.exist?(copied_framework_path)
 >        FileUtils.cp_r(File.join(cached_framework_dir, 'Flutter.framework'), copied_flutter_dir)
 #      end
 #  -------------------------------------------

/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:318:in `rescue in block in from_ruby'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:312:in `block in from_ruby'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:50:in `instance_eval'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:50:in `initialize'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:310:in `new'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:310:in `from_ruby'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-core-1.10.1/lib/cocoapods-core/podfile.rb:276:in `from_file'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-1.10.1/lib/cocoapods/config.rb:205:in `podfile'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-1.10.1/lib/cocoapods/command.rb:160:in `verify_podfile_exists!'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-1.10.1/lib/cocoapods/command/install.rb:46:in `run'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/claide-1.0.3/lib/claide/command.rb:334:in `run'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-1.10.1/lib/cocoapods/command.rb:52:in `run'
/Users/zzz/.rvm/rubies/ruby-2.7.2/lib/ruby/gems/2.7.0/gems/cocoapods-1.10.1/bin/pod:55:in `<top (required)>'
/Users/zzz/.rvm/rubies/ruby-2.7.2/bin/pod:23:in `load'
/Users/zzz/.rvm/rubies/ruby-2.7.2/bin/pod:23:in `<main>'
/Users/zzz/.rvm/gems/ruby-2.7.2/bin/ruby_executable_hooks:22:in `eval'
/Users/zzz/.rvm/gems/ruby-2.7.2/bin/ruby_executable_hooks:22:in `<main>'

Error running pod install
Error launching application on iPhone 12 Pro.

$ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.0.6, on macOS 11.4 20F71 darwin-x64, locale zh-Hans-CN)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.2)
[✓] Xcode - develop for iOS and macOS
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.2)
[✓] IntelliJ IDEA Ultimate Edition (version 2017.3.2)
[✓] VS Code (version 1.58.0)
[✓] Connected device (3 available)

• No issues found!