TIBQuery.FreeStatement error!

[__Newbie__]

I am encountering error below:

TIBQuery.FreeStatement:
-invalid statement handle

I have setup a IBdatabase, IBTransaction, IBQuery => DataSet => dbGrid.
And on design-time as well as runtime they are properly connected with data retrieved.
Only after exiting the application, that error occured. What seems to be wrong?

Thanks for any help.