You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: backend_pubsub.go
    path of file: ./repos/watermill/components/requestreply
    
    the code of the file until where you have to start completion: package requestreply

import (
	"context"
	"fmt"
	"time"

	"github.com/ThreeDotsLabs/watermill"
	"github.com/ThreeDotsLabs/watermill/message"
	"github.com/hashicorp/go-multierror"
	"github.com/pkg/errors"
)

// PubSubBackend is a Backend that uses Pub/Sub to transport commands and repl
    
func (p *PubSubBackendConfig) Validate() error {
	var err error

	if p.Publisher == nil {
		err = multierror.Append(err, errors.New("publisher cannot be nil"))
	}
	if p.SubscriberConstructor == nil {
		err = multierror.Append(err, errors.New("subscriber constructor cannot be nil"))
	}
	if p.GeneratePublishTopic == nil {
		err = multierror.Append(err, errors.New("GeneratePublishTopic cannot be nil"))
	}
	if p.GenerateSubscribeTopic == nil {
		err = multierror.Append(err, errors.New("GenerateSubscribeTopic cannot be nil"))
	}

	return err
}