Problem with child page deselect page type defaults.

Permalink
In pages and themes > page types > defaults;

I was going to delete a block that I no longer wanted. The first thing I did was try to "deselect" it from its child pages. When I try this I get this message;
"Fatal error: Call to a member function deleteBlock() on a non-object in /home/burtsgh8/public_html/updates/concrete5.4.0.3/concrete/startup/process.php on line 160"

Any ideas on why that is?