Getting error using GUIDE

It is a imple code. I just added code to the btn_Graficar. I still don't know how to use the software, but I get this error every time I add code to the callbacks when i use GUIDE

function varargout = untitled(varargin)
% UNTITLED MATLAB code for untitled.fig
%      UNTITLED, by itself, creates a new UNTITLED or raises the existing
%      singleton*.
%
%      H = UNTITLED returns the handle to a new UNTITLED or the handle to
%      the existing singleton*.
%
%      UNTITLED('CALLBACK',hObject,eventData,handles,...) calls the local
%      function named CALLBACK in UNTITLED.M with the given input arguments.
%
%      UNTITLED('Property','Value',...) creates a new UNTITLED or raises the
%      existing singleton*.  Starting from the left, property value pairs are
%      applied to the GUI before untitled_OpeningFcn gets called.  An
%      unrecognized property name or invalid value makes property application
%      stop.  All inputs are passed to untitled_OpeningFcn via varargin.
%
%      *See GUI Options on GUIDE's Tools menu.  Choose "GUI allows only one
%      instance to run (singleton)".
%
% See also: GUIDE, GUIDATA, GUIHANDLES

% Edit the above text to modify the response to help untitled

% Last Modified by GUIDE v2.5 03-Jun-2021 12:15:13

% Begin initialization code - DO NOT EDIT
gui_Singleton = 1;
gui_State = struct('gui_Name',       mfilename, ...
                   'gui_Singleton',  gui_Singleton, ...
                   'gui_OpeningFcn', @untitled_OpeningFcn, ...
                   'gui_OutputFcn',  @untitled_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
% End initialization code - DO NOT EDIT


% --- Executes just before untitled is made visible.
function untitled_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 untitled (see VARARGIN)

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

% Update handles structure
guidata(hObject, handles);

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


% --- Outputs from this function are returned to the command line.
function varargout = untitled_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;



function txtFunciom_Callback(hObject, eventdata, handles)
% hObject    handle to txtFunciom (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of txtFunciom as text
%        str2double(get(hObject,'String')) returns contents of txtFunciom as a double


% --- Executes during object creation, after setting all properties.
function txtFunciom_CreateFcn(hObject, eventdata, handles)
% hObject    handle to txtFunciom (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end



function txtA_Callback(hObject, eventdata, handles)
% hObject    handle to txtA (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of txtA as text
%        str2double(get(hObject,'String')) returns contents of txtA as a double


% --- Executes during object creation, after setting all properties.
function txtA_CreateFcn(hObject, eventdata, handles)
% hObject    handle to txtA (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end



function txtB_Callback(hObject, eventdata, handles)
% hObject    handle to txtB (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'String') returns contents of txtB as text
%        str2double(get(hObject,'String')) returns contents of txtB as a double


% --- Executes during object creation, after setting all properties.
function txtB_CreateFcn(hObject, eventdata, handles)
% hObject    handle to txtB (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    empty - handles not created until after all CreateFcns called

% Hint: edit controls usually have a white background on Windows.
%       See ISPC and COMPUTER.
if ispc && isequal(get(hObject,'BackgroundColor'), get(0,'defaultUicontrolBackgroundColor'))
    set(hObject,'BackgroundColor','white');
end


% --- Executes on button press in btnGraficar.
function btnGraficar_Callback(hObject, eventdata, handles)
% hObject    handle to btnGraficar (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

a=eval (get(handles.txtA,"String"));
b=eval (get(handles.txtB,"String"));
x=a:0.5:b;
y=eval(get(handles.txtFuncion,"String"));
plot(x,y);

I get the next error

Error using matlab.ui.control.UIControl/get
Invalid input argument type to 'get'.  Type 'help get' for options.

Error in untitled>btnGraficar_Callback (line 151)
a=eval (get(handles.txtA,"String"));

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

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

Error in matlab.graphics.internal.figfile.FigFile/read>@(hObject,eventdata)untitled('btnGraficar_Callback',hObject,eventdata,guidata(hObject))
 
Error while evaluating UIControl Callback.

Answer 1

[Voss]

Try

get(handles.txtA,'String')

etc., with single quotes instead of double quotes.

Answer 2

[Image Analyst]

Instead of

a=eval (get(handles.txtA,"String"));
b=eval (get(handles.txtB,"String"));
x=a:0.5:b;

do this:

a = str2double(handles.txtA.String);
b = str2double(handles.txtB.String);
x = a : 0.5 :  b;