Plugins: Check if `_error_nonce` is set before attempting to verify it.

This avoids a PHP notice on the Plugins screen when `error` query string is added to the URL.

Props wponlinesupport.
Fixes #43876.
Built from https://develop.svn.wordpress.org/trunk@49683


git-svn-id: http://core.svn.wordpress.org/trunk@49406 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/plugins.php

@@ -641,7 +641,9 @@ if ( isset( $_GET['error'] ) ) :
 	<div id="message" class="error"><p><?php echo $errmsg; ?></p>
 	<?php
 
-	if ( ! isset( $_GET['main'] ) && ! isset( $_GET['charsout'] ) && wp_verify_nonce( $_GET['_error_nonce'], 'plugin-activation-error_' . $plugin ) ) {
+	if ( ! isset( $_GET['main'] ) && ! isset( $_GET['charsout'] )
+		&& isset( $_GET['_error_nonce'] ) && wp_verify_nonce( $_GET['_error_nonce'], 'plugin-activation-error_' . $plugin )
+	) {
 		$iframe_url = add_query_arg(
 			array(
 				'action'   => 'error_scrape',

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49681';
+$wp_version = '5.7-alpha-49683';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.