Question

Record id 116 doesn’t exist so it should return nil to @conversation.
I tried to make it redirect when it gets nil, but it still shows the error when I access example.com/messages/show?id=116 .

The error is

undefined method `is_participant?’ for nil:NilClass

I definitely see ‘is_participant’ method existing in
/usr/local/lib/ruby/gems/1.9.1/gems/mailboxer-0.7.0/app/models/conversation.rb

messages_controller.rb

def show
  @conversation = Conversation.find_by_id(params[:id])

  unless @conversation.is_participant?(current_user)
    flash[:alert] = "You do not have permission to view that conversation."
    redirect_to :controller => 'messages', :action => 'received'
  end

  @messages = Message.find_by_id(params[:id])
  current_user.read(@conversation)    
end

Answer 1

You need to check that @conversation is not nil before you call a method on it. Try

unless @conversation.present? && @conversation.is_participant?(current_user)