Confusing 'not working' button - grey it out until the action is possible

[anvabr]

Problem description

A naive user might try to click 'Add document' button when creating a tag without knowing about the necessary pre-conditions, and face a 'not working' situation.

Step to reproduce

1. Click 'Add document' button in the new tag dialogue as per the image below

Expected behavior

The precondition for this action to work is created and published schema for the document. The button must be greyed out until the user has done so, and the greyed out button should display a tooltip with "Create a publish document schema first" text when a mouse cursor is positioned on top of it.