Administration: Ensure get_current_screen() returns WP_Screen or null.

This adds a check that the `$current_screen` global is not only defined, but also has the correct type.

Follow-up to [15746].

Props marian1, mayanktripathi32, abcd95, im3dabasia1, SergeyBiryukov.
Fixes #62562.

git-svn-id: https://develop.svn.wordpress.org/trunk@61484 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/screen.php

@@ -224,7 +224,7 @@ function add_screen_option( $option, $args = array() ) {
 function get_current_screen() {
 	global $current_screen;
 
-	if ( ! isset( $current_screen ) ) {
+	if ( ! $current_screen instanceof WP_Screen ) {
 		return null;
 	}
 

tests/phpunit/tests/admin/includesScreen.php

@@ -451,6 +451,19 @@ public function test_options() {
 		$this->assertSame( $screen->get_options(), array() );
 	}
 
+	/**
+	 * @ticket 62562
+	 */
+	public function test_get_current_screen_type() {
+		global $current_screen;
+
+		set_current_screen( 'edit.php' );
+		$this->assertInstanceOf( 'WP_Screen', get_current_screen() );
+
+		$current_screen = new stdClass();
+		$this->assertNull( get_current_screen() );
+	}
+
 	public function test_in_admin() {
 		set_current_screen( 'edit.php' );
 		$this->assertTrue( get_current_screen()->in_admin() );