As lucamar and I told you...
You access frmAstCont in the OnCreate event of Form1. Looking at the project file you will see that at this time the frmAstCont is not yet created and thus nil.
You can use the OnShow or OnActivate events instead. On the other hand, these events may be fired later during execution of the program again. To prevent unwanted side-effects when this happens, I often implement a method "BeforeRun" of the mainform which is called in the project unit immediately before Application.Run.
type
TForm1 = class(TForm)
...
public
procedure BeforeRun;
...
end;
procedure TForm1.BeforeRun;
begin
_MinMaxFechaContable;
end;
----------
program Project;
...
begin
....
Application.Initialize;
Application.CreateForm(TForm1, Form1);
Application.CreateForm(TfrmAstCont, frmAstCont);
Form1.BeforeRun;
Application.Run;
end.
This way the crash due to nonexisting frmAstConCont is prevented. But as you'll notice there is another problem with your comboboxes - I'll leave this to you to find a solution...