Field UI module is a dependency for settings form

Problem/Motivation

With field_ui core module disabled you cannot save the settings form (/admin/config/content/cancel-button), there are two errors, "Form mode" and "View mode" displays "The path ... is either invalid or you do not have access to it."

Proposed resolution

Adding a check for the enabled field_ui module, and hiding "Form mode" and "View mode" from the settings form if the module is not enabled.