Add Header in WordPress Administration Screen → Too view arguments Message

Prerequisites/What you want to achieve

I added Header to the Appearance item on the management screen in WordPress and clicked on the Header.
If it goes well, you should be able to verify that the current main image on the front page is being previewed.

However, this time the following message came out and the preview could not be confirmed.

I would like to improve it so that the preview of the main image is displayed.

インストール I installed WordPress 5.1.1, and the reference book is 4.x.Version mismatch.I would like to know how to improve it.

"* I wrote ""%s"" in the image insertion area (as per reference book)."Regarding that, I received the following points from another question site.
"The URL I set to default-image contains \"%s\", but I think I'll replace it with something and then call add_theme_support."

I'm in the middle of investigating a solution.I would like to solve this as well.

Header Program (function.php)

<?php

add_theme_support(
  'custom-header',
  array(
    'width' = > 950,
    'height' = > 295,
    'header-text' = > false,
    'default-image' = > '%s/images/top/main_image.png',
  )
);

Error Message Occurring

Warning:sprintf():Too new arguments in Home Page Address /wp-includes/theme.php on line 915

Tried

I thought the line 915 in the wp-includes folder >theme.php might be strange in FTP of my partner server (X server), but there were no lines 915 in the first place.

Supplementary Information 1 (for example, reference books)

We have added headers as per the text "WordPress Textbook Ver.4.x Compatible Version" (P58)

The theme of the appearance was not uploaded into FTP on the X server, but directly into WordPress.

Supplement 2 (tried after being answered on another question site)

I changed it to the following referring to the official page of wordpress, but the situation was the same (incorrect screen after error message).
*We are currently investigating the %s you pointed out.

<?php

// Custom Header
add_theme_support(
  'custom-header',
  $defaults=array(
    'default-image' = > '%s/images/top/main_image.png',
    'width' = > 950,
    'height' = > 295,
    'header-text' = > false,
  )
);
add_theme_support('custom-header',$defaults);