Fix PHP fatal error

Author: girishpanchal30

.github/workflows/test-php.yml

@@ -13,7 +13,7 @@ jobs:
       - name: Setup PHP version
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '7.2'
+          php-version: '7.4'
           extensions: simplexml
       - name: Checkout source code
         uses: actions/checkout@v2
@@ -49,7 +49,7 @@ jobs:
       - name: Setup PHP version
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '7.2'
+          php-version: '7.4'
           extensions: simplexml, mysql
           tools: phpunit-polyfills
       - name: Checkout source code

tests/test-ajax.php

@@ -37,7 +37,7 @@ class Test_Visualizer_Ajax extends WP_Ajax_UnitTestCase {
 	/**
 	 * Set up.
 	 */
-	public function setUp() {
+	public function setUp(): void {
 		parent::setUp();
 		$this->admin_user_id = $this->factory->user->create(
 			array(