Rails 4: Devise.secret_key was not set - ruby-on-rails

Question

This seems to be a popular issue, since I found many similar questions on Stack Overflow:
Devise Token Auth error: Devise.secret_key was not set
Devise Secret Key was not set
Mina and Devise.secret_key was not set
However, none of the above seems to help fix my problem.
Here is my situation:
I installed Font Awesome with the font-awesome-rails gem, and it was working locally, but whenever I deployed to Heroku, Font Awesome icons were no longer appearing.
So, I looked for answers to fix this problem, and found out that I needed to run RAILS_ENV=production bundle exec rake assets:precompile to make things work.
However, when I run this command, I get the following error in Terminal:
rake aborted!
Devise.secret_key was not set. Please add the following to your Devise initializer:
config.secret_key = '—— MY_SECRET_KEY_HERE ———'
Please ensure you restarted your application after installing Devise or setting the key.
As per the solutions offered in the questions mentioned at the beginning of my question, I do haveconfig.secret_key = ENV['DEVISE_SECRET_KEY'] if Rails.env.production? in devise.rb
Also, as recommended here, I did run heroku config:set DEVISE_SECRET_KEY='—— MY_SECRET_KEY_HERE ———' -- app name_of_my_app_here.
Because I was not sure, I also ran heroku config:set DEVISE_SECRET_KEY=—— MY_SECRET_KEY_HERE ——— -- app name_of_my_app_here (without single quotes around my secret key).
But none of these things worked.
I keep getting the same error.
What am I doing wrong?

Answer 1

I found a solution to this problem.
Instead of keeping on struggling with the font-awesome-rails gem, I uninstalled it and replaced it with font-awesome-sass, which is the official Font Awesome gem for rails.
Following the guidelines offered here (section: EASY: SASS Ruby Gem), I then ran bundle and added:
#import "font-awesome-sprockets";
#import "font-awesome";
to my application.scss file.
I also removed the old #import "font-awesome.css"; line from the very same file.
Now, Font Awesome works both locally and on Heroku.