What is this error about after am trying to indice an image out of a uibutton and uicontrols ?

I use this script

bg = uibuttongroup('Visible','off',...
                  'Position',[0 0 .2 1],...
                  'SelectionChangeFcn',@Do_plot, ...
                  'Tag', 'BG1');
handles.BG1 = bg;
guidata(bg, handles);
              
r1 = uicontrol(bg,'Style',...
                  'radiobutton',...
                  'String','Option 1',...
                  'Position',[10 350 100 30],...
                  'HandleVisibility','off');
              
r2 = uicontrol(bg,'Style','radiobutton',...
                  'String','Option 2',...
                  'Position',[10 250 100 30],...
                  'HandleVisibility','off');
              % Make the uibuttongroup visible after creating child objects. 
bg.Visible = 'on';

And this function in the end

function Do_plot(hObject, event, varargin)
   bg = findobj(gcf, 'Tag', 'BG1');
   sel = bg.SelectedObject;
   if isempty(sel)
       return;  %no buttons selected
   end
   sel_string = sel.String;
   switch sel_string
       case 'Option 1'
           bg.Value = croppedImage;
       case 'Option 2'
           bg.Value = J;
   end
   bg= double(bg);
end

And I get that error

Error using uint8
Conversion to uint8 from matlab.ui.container.ButtonGroup is not possible.
Error in ruller (line 181)
figure,imagesc(uint8(Img),[0 255]),colormap(gray),axis off;axis equal 
>> 

Why is that?

[Stelios Fanourakis]

Or this error?

Error using contour (line 48)
Z must be at least a 2x2 matrix.
Error in ruller (line 193)
hold on,[c,h] = contour(phi,[0 0],'r','linewidth',1); hold off