Eaccesviolation during Drag&Drop


I have 2 Tlistview.
Sometimes, during a drag&drop, there is an EaccesViolation, always at the same adress.
Delphi says 'Reading adress FFFFFFFF'.
The problem is that the bug arrives before the mouseUp, so my code is not executed, so I'm not able to understand why there is this error.
I have tryied to search at the adress 0042F024 (adress of the Eacces violation).
Delphi is unable to find something.

An idea?

Thanks' Patrice.