Cocoapods post install script that ensures NSPrincipalClass is copied across to Info.plist of nominated pods. Without this NSPrincipalClass will be nil.

Podfile

# Post install script that ensures that NSPrincipalClass 
# is transferred from original info.plist to the pod generated info.plist.
post_install do |installer|

  # Frameworks and their principal classes dictionary
  frameworks_classes = { 
    'FrameworkA' => 'FrameworkAPrincipalClassName',
    'FrameworkB' => 'FrameworkBPrincipalClassName'
  }

  # Scan directory tree
  frameworks_classes.each do |framework_name, framework_class|
    directory = installer.config.project_pods_root
    Dir.foreach(directory) do |path|
      if path.to_s == 'Target Support Files'
        full_path = directory + path
        if File.directory?(full_path)
          puts full_path
          Dir.foreach(full_path) do |path2|
            if path2.to_s == framework_name
              full_path = full_path + path2
              Dir.foreach(full_path) do |path3|
                if path3.to_s == 'Info.plist'
                  info_plist_path = full_path + path3
                  # Open Info.plist and modify it
                  if File.exist?(info_plist_path)                
                      text = File.read(info_plist_path)
                      new_contents = text.gsub(/(<key>NSPrincipalClass<\/key>\s*<string>)()(<\/string>)/, '\1' + framework_class + '\3')
                      File.open(info_plist_path, "w") {|file| file.puts new_contents }
                  end
                end
              end
            end
          end
        end
      end
    end  
  end
end