Update Excel2007.php

Author: toyi

Classes/PHPExcel/Reader/Excel2007.php

@@ -1688,7 +1688,7 @@ public function load($pFilename)
 
                     if ((!$this->readDataOnly) || (!empty($this->loadSheetsOnly))) {
                         // active sheet index
-                        $activeTab = intval($xmlWorkbook->bookViews->workbookView["activeTab"]); // refers to old sheet index
+                        $activeTab = intval($xmlWorkbook->bookViews->workbookView["activeTab"] ?? 0); // refers to old sheet index
 
                         // keep active sheet index if sheet is still loaded, else first sheet is set as the active
                         if (isset($mapSheetId[$activeTab]) && $mapSheetId[$activeTab] !== null) {