How to recongnize a field in GUI

I was about to practice my GUI exercise. For my GUI, I added the following codes in GUI opening function so that all the components recognized the function.

GUI code

function Exercise2_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 Exercise2 (see VARARGIN)

% Choose default command line output for Exercise2
handles.output = hObject;

% Update handles structure
guidata(hObject, handles);

x_str=get(handles.XEditStr,'string');
y_str=get(handles.YEditStr,'string');

x=str2num(x_str);
y=eval(y_str);

handles.Line_handle=feval('plot',x,y);
CurrentLineColor=get(handles.Line_handle,'color');

set(handles.RSlid,'value',CurrentLineColor(1));
set(handles.GSlid,'value',CurrentLineColor(2));
set(handles.BSlid,'value',CurrentLineColor(3));

set(handles.RSlidValTxt,'string',num2str(CurrentLineColor(1)));
set(handles.GSlidValTxt,'string',num2str(CurrentLineColor(2)));
set(handles.BSlidValTxt,'string',num2str(CurrentLineColor(3)));

However, It did not work and gave an error as follows.

Error Message

Reference to non-existent field 'Line_handle'.

Error in GUIDemo>RSlid_Callback (line 153)
CurrentColorVal=get(handles.Line_handle,'color');

Error in gui_mainfcn (line 96)
        feval(varargin{:});

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

Error in @(hObject,eventdata)GUIDemo('RSlid_Callback',hObject,eventdata,guidata(hObject))

Error while evaluating uicontrol Callback

How to solve this problem and work my GUI correctly? Please give me your answers. Thank you.

Answer 1

[Walter Roberson]

The error message you show is from a callback whose code you do not show.

However, we predict that you should move

% Update handles structure
guidata(hObject, handles);

to after the code that you posted.