Fix return type hint (#947)

CHANGELOG.md

@@ -17,6 +17,7 @@ and this project adheres to [Semantic Versioning](https://semver.org).
 
 - Fix handling for escaped enclosures and new lines in CSV Separator Inference
 - Fix MATCH an error was appearing when comparing strings against 0 (always true)
+- Fix return type hint
 
 ## [1.6.0] - 2019-01-02
 

src/PhpSpreadsheet/Spreadsheet.php

@@ -715,7 +715,7 @@ class Spreadsheet
      *
      * @param string $pName Sheet name
      *
-     * @return Worksheet
+     * @return null|Worksheet
      */
     public function getSheetByName($pName)
     {