How to add bridging header to Flutter Plugin

I'm trying to create Flutter plugin, which uses Swift and has one pod dependency (NearbyMessages)

First of all, I've added it to .podspec

#
# To learn more about a Podspec see http://guides.cocoapods.org/syntax/podspec.html
#
Pod::Spec.new do |s|
  s.name             = 'flutter_nearby_messages'
  s.version          = '0.0.1'
  s.summary          = 'A new flutter plugin project.'
  s.description      = <<-DESC
A new flutter plugin project.
                       DESC
  s.homepage         = 'http://example.com'
  s.license          = { :file => '../LICENSE' }
  s.author           = { 'Your Company' => 'email@example.com' }
  s.source           = { :path => '.' }
  s.source_files = 'Classes/**/*'
  s.public_header_files = 'Classes/**/*.h'
  s.dependency 'Flutter'
  //************ I've added this lines ***************
  s.dependency 'NearbyMessages' 
  s.xcconfig = { 'SWIFT_OBJC_BRIDGING_HEADER' => 'Classes/bridging_header.h' }
  //********* ^^ I've added this lines ^^ ************
  s.static_framework = true
  s.ios.deployment_target = '8.0'
end

After pod install and initial empty project built, everything seems to be fine

I've tried using it in Objective-C:

#import "FlutterNearbyMessagesPlugin.h"
#import <NearbyMessages/GNSMessages.h>
#import <flutter_nearby_messages/flutter_nearby_messages-Swift.h>

@implementation FlutterNearbyMessagesPlugin
+ (void)registerWithRegistrar:(NSObject<FlutterPluginRegistrar>*)registrar {    
    GNSMessageManager *messageManager =
    [[GNSMessageManager alloc] initWithAPIKey:@"API_KEY"];
  [SwiftFlutterNearbyMessagesPlugin registerWithRegistrar:registrar];
}
@end

And it works, but when I try:

public class SwiftFlutterNearbyMessagesPlugin: NSObject, FlutterPlugin {
    private var client: GNSMessageManager? = nil

XCode says Use of undeclared type GNSMessageManager

I understand, that I need bridging header, and I've already tried to creating it, but it seems not to be linked at all

So my question is, how to add bridging header to flutter plugin?

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