Description of the problem
We have a system with four MSM sites. Some use Structure, some don't. On saving an entry where Structure isn't set up I get the following error:

Deprecated

base64_decode(): Passing null to parameter #1 ($string) of type string is deprecated

ee/ExpressionEngine/Addons/structure/sql.structure.php, line 1437

Severity: E_DEPRECATED

The entry still saves but the success screen isn't shown, instead the error blocks the output.

Environment Details:

• Version: 7.5.7
• PHP Version 8.3.19
• MySQL Version 5.7.42
• OS: Linux
• Web Server: Apache

Possible Solution
Replace line 1437 of sql.structure.php with the following:

if(is_string($pages_array))
{
		$all_pages = unserialize(base64_decode($pages_array['site_pages']));
}
else
{
	$all_pages = array();
}

There could be a better solution, but this seems to work for me.

Additional context
Our site has been updated from Version 2 via intermediate versions, so it's possible that this is the source of the issue.