The type patcher script also fixes some return statements automatically to deal with void/null differences. However, the script is a bit too eager and also patches return statements of closures inside methods:

publicfunctionsomething():void{$someHandler =function (): ?string {if (...) {returnnull;        }    };}

Statements like this were patched toreturn;, causing a PHP fatal error.

Note: I didn't find any test for the return statement patching, so I also didn't write one for this bug. However, this bug is discovered by our own CI in#49342, so I would say we've covered the fix :)