type
  TJob = string;
  TJobList = Class
  Private
    fList : TThreadList;
  Public
    Function GetNextJob : TJob;
    Procedure AddJob (aJob : TJob);
    End;

  TMainForm = class(TForm)
    btnStartThreads: TButton;
    ...

implementation

{$R *.dfm}

Function TJobList.GetNextJob : TJob;
Var
  l :TList;

Begin
  l := fList.LockList;
  Try
    if l.Count = 0 Then
      Result := Nil
    else <<<<< Fehler 1
    begin
      Result := l[l.count - 1]; <<<<< Fehler 2
      l.delete (l.count - 1);
      End;
  Finally
     fList.unlockList;
     End;
End;

Procedure TJobList.AddJob (aJob : TJob);
Var
  l :TList;

Begin
  l := fList.LockList;
  Try
    l.Add (aJob); <<<<< Fehler 3
  Finally
    fList.unlockList;
    End;
End;