Why can Invalid pointer operation occur in Pascal?

Question

Pascal

There is a function in Pascal-like syntax in this line, when working, an error occurs Invalid pointer operation from the description it is clear that:if oNode <> nil then FreeObject(oNode);

204 Invalid pointer operation
You will get this if you call Dispose or Freemem with an invalid pointer (notably, Nil).

That is, nil is only one of the possible options, but in this code FreeObject is a priori called only when the object is not nil, and therefore the question is what else could be wrong with the object, that it cannot be cleared and how to check it?
ps perhaps the reason is that this object was assigned to one of the elements of the list, which was cleared before clearing this object, that is, the object seems to exist, but the data no longer exists

Answer 1

Most likely, this is how it is: calling "cleanup" twice on the same object.

Answer 2

ps perhaps the reason is that this object was assigned to one of the elements of the list, which was cleared before clearing this object, that is, the object seems to exist, but the data no longer exists

Exactly.
It makes sense to use the FreeAndNil function inside the FreeObject implementation.