Fixed cannot save posts when select_tree has required attribute.

Link: https://metabox.io/support/topic/error-in-select-tree-taxonomy/

The taxonomy has some term nested in 3 levels, but it also has other nested terms in only 2 levels. Then the validation of the abligatory field fails because apparently it is waiting for a third level, even though it should not be like that.

This fix set/unset the 'required' attribute correspondingly when select
is visible/hidden (similar to Conditional Logic). So doesn't affect the
valiation when saving.

js/select-tree.js

@@ -1,19 +1,38 @@
 jQuery( function ( $ ) {
 	'use strict';
 
+	function setInitialRequiredProp() {
+		var $this = $( this ),
+			required = $this.prop( 'required' );
+
+		if ( required ) {
+			$this.data( 'initial-required', required );
+		}
+	}
+
+	function unsetRequiredProp() {
+		$( this ).prop( 'required', false );
+	}
+
+	function setRequiredProp() {
+		var $this = $( this );
+
+		if ( $this.data( 'initial-required' ) ) {
+			$this.prop( 'required', true );
+		}
+	}
+
 	function update() {
 		var $this = $( this ),
 			val = $this.val(),
 			$selected = $this.siblings( "[data-parent-id='" + val + "']" ),
-			$notSelected = $this.parent().find( '.rwmb-select-tree' ).not( $selected );
+			$notSelected = $this.siblings().not( $selected );
 
-		$selected.removeClass( 'hidden' );
-		$notSelected
-			.addClass( 'hidden' )
-			.find( 'select' )
-			.prop( 'selectedIndex', 0 );
+		$selected.removeClass( 'hidden' ).find( 'select' ).each( setRequiredProp );
+		$notSelected.addClass( 'hidden' ).find( 'select' ).each( unsetRequiredProp ).prop( 'selectedIndex', 0 );
 	}
 
+	$( '.rwmb-select-tree select' ).each( setInitialRequiredProp );
 	$( '.rwmb-input' )
 		.on( 'change', '.rwmb-select-tree select', update )
 		.on( 'clone', '.rwmb-select-tree select', update );