Matlab - problema con listbox

function varargout = listbox7(varargin)

gui_Singleton = 1;

gui_State = struct('gui_Name',       mfilename, ...

                   'gui_Singleton',  gui_Singleton, ...

                   'gui_OpeningFcn', @listbox7_OpeningFcn, ...

                   'gui_OutputFcn',  @listbox7_OutputFcn, ...

                   'gui_LayoutFcn',  [] , ...

                   'gui_Callback',   []);

if nargin && ischar(varargin{1})

    gui_State.gui_Callback = str2func(varargin{1});

end

if nargout

    [varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});

else

    gui_mainfcn(gui_State, varargin{:});

end

function listbox7_OpeningFcn(hObject, eventdata, handles, varargin)

nombres=ls;

set(handles.listbox1,'string',nombres);

handles.output = hObject;

guidata(hObject, handles);

function varargout = listbox7_OutputFcn(hObject, eventdata, handles)

varargout{1} = handles.output;

function listbox1_Callback(hObject, eventdata, handles)

 contents=cellstr(get(hObject,'String')) ;

 nombre=contents{get(hObject,'Value')};

 fid=fopen('archivocaballero.txt','w');

 fprintf(fid,'%s \n',nombre);

 fclose(fid);

set(handles.listbox1,'string',' ');

 nombres=dir(fullfile(pwd));

 set(handles.listbox1,'string', {nombres.name});