Issue
When trying to open a workflow in Nintex Workflow for Office 365 that contains 'Run Component Workflow' action, the following error may occur:
Error opening a workflow. Object reference not set to an instance of an object
Error opening a workflow. Object reference not set to an instance of an object
Resolution
The issue has been fixed on the latest version of Nintex Workflow for Office 365 Apps that is being released by the end of July 2018. Make sure that the site has the latest version of Nintex Apps.