Bug fixes / Support for WP5.8 widget block editor

【Contents of the fix】
■Support for the WordPress 5.8 widget block editor
■Fixed a bug that an error could occur when using the fixed page template “Design Page 2” and leaving the “Text + Background Image” heading blank.
■Fixed character encoding of some files.

【Modified files】
languages/ja.mo
languages/ja.po
functions.php
page-design1.php
page-design2.php
style.css