undefined method `set' for nil:NilClass in Rails even though similar code works in irb - ruby-on-rails

Question

The following code works fine in IRB (Interactive Ruby Shell):
require 'prometheus/client'
prometheus = Prometheus::Client.registry
begin
#requests = prometheus.gauge(:demo, 'Random number selected for this users turn.')
rescue Prometheus::Client::Registry::AlreadyRegisteredError => e
end
#requests.set({name: "test"}, 123)
test = #requests.get name: "test"
puts 'output: ' + test.to_s
2.4.0 :018 > load 'test.rb'
output: 123.0
=> true
2.4.0 :019 >
However, when I put the same code into my Ruby on Rails controller, the second time the user uses the application, the following error is returned:
undefined method `set' for nil:NilClass
Can someone tell me when I'm doing wrong? Thank you.
require 'prometheus/client'
class RandomnumbersController < ApplicationController
def index
#randomnumbers = Randomnumber.order('number DESC').limit(8)
#counter = 0
end
def show
#randomnumber = Randomnumber.find(params[:id])
end
def new
end
def create
#randomnumber = Randomnumber.new(randomnumber_params)
prometheus = Prometheus::Client.registry
begin
#requests = prometheus.gauge(:demo, 'Random number selected for this users turn.')
rescue Prometheus::Client::Registry::AlreadyRegisteredError => e
end
#requests.set({name: "test"}, 123)
test = #requests.get name: "test"
#randomnumber.save
redirect_to #randomnumber
end
private
def randomnumber_params
params.require(:randomnumber).permit(:name, :number)
end
end

Answer 1

Because there is no #requests for :demo argument.
When ORM cannot find any info in db it returns nil (NilClass)
and You're trying to do:
#requests.set({name: "test"}, 123)
it's interpreted like:
nil.set({name: "test"}, 123)
why it's causes this issue in second time?
cuz Your code changes #requests name attribute to be test and seems like :demo is not test or maybe in another part of Your app You're replacing/deleting data in database that makes: #requests = prometheus.gauge(:demo, 'Random number selected for this users turn.') to return nil
Solution:
in code level add this fixes to avoid such unpredictable situations (check for nil) :
unless #requests.nil?
#requests.set({name: "test"}, 123)
test = #requests.get name: "test"
end