The stand-alone gui gives an error while the main guide I call is running, how can I fix it?

Question

StepResponse.fig

function OK_Callback(hObject, eventdata, handles)
global x y
syms s;

num = str2sym(get(handles.num1, 'String'));
den= str2sym(get(handles.den1, 'String') );
sys = tf(sym2poly(num), sym2poly(den));
x= eval(get(handles.x_time,'String'));
u = ones(size(x));
[y, ~] = lsim(sys, u, x);
plot ( x,y)

This gui , I run it code it is work but I call second gui I have error. Dot indexing is not supported for variables of this type.

" Error in stepresponse>OK_Callback (line 127) num = str2sym(get(handles.num1, 'String'));

Error in gui_mainfcn (line 95) feval(varargin{:});

Error in stepresponse (line 42) gui_mainfcn(gui_State, varargin{:});

Error in matlab.graphics.internal.figfile.FigFile/read>@(hObject,eventdata)stepresponse('OK_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating UIControl Callback. "

Second gui :

 
function listbox2_Callback(hObject, eventdata, handles)

 h= get(hObject,'Value');
  switch h
      case 1
          open Nyquist.fig 
      case 2
          open stepresponse.fig
  end 

I am not solving problem because works sperately but not work together.