I found a half-day error! AV error. Be careful when the component object uses application as the owner.

VaR
Connectionservice: = tconnservice;

Procedure tfrmmain. formcreate (Sender: tobject );
Begin
Connectionservice: = tconnservice. Create (application );
End;

Destructor tfrmmain. Destroy;
Begin
Connectionservice. Free // An error occurred while executing the command because the application is used as the owner.
// Before that, the application automatically releases the connectionservice (releases all applications whose owner is the application)
Inherited;
End;

This will cause an error. frmmain is the main form of the application.