error when try to increment one of the different target in my project

[st-small]

Thank you very much for your plugin. But I have trouble and want to fix it. I have project with multiple targets and each of them has different build number. I want to get current build number of the target and increment it. This is my example:

project = './CoreFrameworkApp/CoreFrameworkApp.xcodeproj'
target = 'Demo'
version = get_build_number_from_plist(xcodeproj: project,
                                        target: target,
                                        build_configuration_name: 'Release').to_i + 1
###################################
platform :ios do
  desc "Builds, achieves and uploads ipa to TestFlight"
  lane :testflight_lane do
    cert
    sigh
    increment_build_number_in_plist(
        build_number: version.to_s,
        target: target,
    )
    gym(
    workspace: "MyProject.xcworkspace",
        scheme: "Demo",
        configuration: "Release",
        clean: true,
        output_directory: "./build",
        output_name: "#{target}.ipa",
    )
    upload_to_testflight(
           skip_submission: true,
           skip_waiting_for_build_processing: true
    )
  end
end

I have error message: 'no implicit conversion of nil into String'. What does it mean? Thank you very much for help.

[st-small]

I'm very sorry. It was my bag. I misunderstood how plugin works. I thought after plugin finished it should update build value in Xcode Info tab. And worried why it didn't updates. But now if I reload my Xcode project it updates successfully. So plugin works perfect! Thank you! My last Fastlane file for example:

default_platform(:ios)
project = './CoreFrameworkApp/CoreFrameworkApp.xcodeproj'
target = 'Demo'
###################################
platform :ios do
  desc "Builds, achieves and uploads ipa to TestFlight"
  lane :testflight_lane do
    cert
    sigh
    increment_build_number_in_plist(xcodeproj: project, target: target)
    gym(
        workspace: "MyProject.xcworkspace",
        scheme: "Demo",
        configuration: "Release",
        clean: true,
        output_directory: "./build",
        output_name: "#{target}.ipa",
    )
    upload_to_testflight(
           skip_submission: true,
           skip_waiting_for_build_processing: true
        )
  end
end