Implementing: javafx.css.Styleable

The id of this PopupControl. This simple string identifier is useful for finding a specific Node within the scene graph. While the id of a Node should be unique within the scene graph, this uniqueness is not enforced. This is analogous to the "id" attribute on an HTML element (CSS ID Specification).

Returns:  the id assigned to this PopupControl using the setId method or null, if no id has been assigned.

@defaultValue null