Old-style Delphi objects have been broken since the release of Delphi 2, perhaps earlier. They do not do inheritance well when they have fields of compiler-managed types, such as string or dynamic arrays. There was a discussion about it in 2004 on comp.lang.pascal.delphi.misc. Here was the code to reproduce it:

type
  TBase = object
  public
    s: string;
  end;

  TDerived = object(TBase)
  end;

procedure test;
var
  obj: TDerived; //okay for TBase!
begin
  assert(obj.s =   ,  uninitialized dynamic variable );
end;

And in fact it s only OK for TBase by accident because of how the function s prologue code happens to be generated. Putting additional code in that function can make it crash anyway.

Indeed, it s exactly as you ve observed — old-style objects don t get initialized properly. Their string fields don t start out holding an empty string; instead, they hold garbage, and so it s not even possible to initialize them yourself without using something like FillChar.

This appears to be due to the variables being local variables. Unit-scope ("global") variables seem to work OK. Variables that are declared at unit scope but only used by the unit s initialization section, or at program scope and used only in the DPR file s main begin-end block, are treated by the compiler as local variables, so they re not set to all-bits-zero like their global counterparts. When you move your variable declaration to a unit but continue to use it in your DPR file, it s elevated to "global" status.

Your TGUI_Element type has a string member called DbgName, and it looks like that s the only string field you have in the type hierarchy. Take that out, or change it to ShortString, and I ll bet your crashes go away, at least temporarily.

Why are you giving all objects individual named constructors instead of making them virtual?

  type   tx = object
                constructor init; virtual;
                end;
         txx = object(tx)
                   constructor init; virtual; // like override in Delphi classes.
                  end;

If you need a visual hierarchy to look at, have a look at Free Vision, it demonstrates nearly every facet of the TP object model

Oops apparantly virtual constructors are not possible in the TP model