Matlab - cerrar ventana con (X)

Hola,
tengo una GUI que llama a un segunda GUI para obtener un valor,
en la segunda GUI tengo un editbox, un botón para aceptar y otro para cancelar, esto funciona perfectamente, pero si en vez de darle a cancelar, le doy al botón de cerrar (X) de la esquina superior derecha me da este error:

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??? Attempt to reference field of non-structure array.

Error in ==> umbralizacion>umbralizacion_OutputFcn at 73
varargout{1} = handles.output;

Error in ==> gui_mainfcn at 197
[varargout{1:nargout}] = feval(gui_State.gui_OutputFcn, gui_hFigure, [], gui_Handles);

Error in ==> umbralizacion at 40
[varargout{1:nargout}] = gui_mainfcn(gui_State, varargin{:});

Error in ==> boceto>Umbralizacion_Callback at 523
umbral = umbralizacion;

Error in ==> gui_mainfcn at 75
feval(varargin{:});

Error in ==> boceto at 42
gui_mainfcn(gui_State, varargin{:});

??? Error while evaluating uimenu Callback.
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En la segunda GUI tengo esto:

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function umbralizacion_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject handle to figure
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
% varargin command line arguments to umbralizacion (see VARARGIN)

% Choose default command line output for umbralizacion
handles.output = [];
% Update handles structure
guidata(hObject, handles);

% UIWAIT makes umbralizacion wait for user response (see UIRESUME)
uiwait(handles.figure1);

% --- Outputs from this function are returned to the command line.
function varargout = umbralizacion_OutputFcn(hObject, eventdata, handles)
% varargout cell array for returning output args (see VARARGOUT);
% hObject handle to figure
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)

% Get default command line output from handles structure
varargout{1} = handles.output;
delete(handles.figure1);
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Alguien podría ayudarme?
Gracias!