fixes #436 Illegal string offset and Invalid argument in FlashMessages

fixes #436 error occurs when trying to add message to messages when
messages is not a valid array, permitted in php7.0 and earlier but not
in php 7.1.  Not reproduced but fixed by adding defensive code to
ensure array is always initialised.

src/Hyyan/WPI/Tools/FlashMessages.php

@@ -132,6 +132,11 @@ public static function getOptionName()
      */
     private static function getMessages()
     {
+        $messages = get_option( static::getOptionName(), array() );
+        if ( ! is_array( $messages ) ) {
+            $messages = array();
+        }
+        return $messages;
         return get_option(static::getOptionName(), array());
     }
 }