By default, all Ninja Forms options are only available to users with the ‘Administrator’ role. This plugin will enable you to set which elements of Ninja Forms different user roles should have access to.
Ninja Forms support line themselves were unaware of this plugin. They were not that helpful in pointing me to the right direction but I'm glad I found this; better than frigging around with custom code.
Cheers to Rapid Web!
P.S. Please confirm compatibility with WordPress 4.9.1.