Taxonomy: Check the result of get_term() in WP_Term_Query::get_terms().

`get_term()` can return `WP_Error` or `null` on failure, so the result should be verified as a `WP_Term` instance before accessing the `count` property.

This commit prevents a PHP warning if `get_term()` returns `null` for a child term:
{{{
Warning: Attempt to read property "count" on null
}}}

Follow-up to [27458], [37572].

Props josephscott, coleatkinson1, kebbet, jakariaistauk, sabernhardt, westonruter, SergeyBiryukov.
Fixes #63877.

git-svn-id: https://develop.svn.wordpress.org/trunk@60661 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/class-wp-term-query.php

@@ -849,7 +849,8 @@ public function get_terms() {
 					if ( is_array( $children ) ) {
 						foreach ( $children as $child_id ) {
 							$child = get_term( $child_id, $term->taxonomy );
-							if ( $child->count ) {
+
+							if ( $child instanceof WP_Term && $child->count ) {
 								continue 2;
 							}
 						}

tests/phpunit/tests/term/query.php

@@ -852,6 +852,53 @@ public function test_null_term_object_should_be_discarded() {
 		$this->assertSameSets( $expected, wp_list_pluck( $found, 'term_id' ) );
 	}
 
+	/**
+	 * Tests that a call to WP_Term_Query::get_terms() does not result in a PHP warning
+	 * when get_term() returns null for a child term.
+	 *
+	 * The warning that we should not see:
+	 * `Warning: Attempt to read property "count" on null`.
+	 *
+	 * @ticket 63877
+	 */
+	public function test_null_child_term_should_not_throw_warning() {
+		register_taxonomy(
+			'wptests_tax',
+			'post',
+			array(
+				'hierarchical' => true,
+			)
+		);
+
+		$t1 = self::factory()->term->create(
+			array(
+				'taxonomy' => 'wptests_tax',
+			)
+		);
+
+		$t2 = self::factory()->term->create(
+			array(
+				'taxonomy' => 'wptests_tax',
+				'parent'   => $t1,
+			)
+		);
+
+		$this->term_id = $t2;
+
+		add_filter( 'get_term', array( $this, 'filter_term_to_null' ) );
+		$q = new WP_Term_Query(
+			array(
+				'taxonomy'   => 'wptests_tax',
+				'hide_empty' => true,
+				'fields'     => 'ids',
+			)
+		);
+		remove_filter( 'get_term', array( $this, 'filter_term_to_null' ) );
+
+		$this->assertIsArray( $q->terms, 'The result should be an array.' );
+		$this->assertEmpty( $q->terms, 'The result should be empty.' );
+	}
+
 	public function filter_term_to_null( $term ) {
 		if ( $this->term_id === $term->term_id ) {
 			return null;