Stop interrupt button & delete line from graph error at GUI

Hi. The link below is the snapshot of my GUI.

http://www.imgplace.com/viewimg440/3707/63383235.png

The error appears when I pressed "Stop" or "Reset" to stop the yellow needle from moving at the graph. Below is the error message from the MATLAB command windows: ??? Error using ==> delete Invalid handle object.

Error in ==> placeT at 164 delete(PPIline)

Now I suspect the error comes from "Generate" button, "Stop" button, or "placeT" callfunction. Below are the snippet of my codes for the 3 functions above:

This is the "Generate Target" button

function genTarget_Callback(hObject, eventdata, handles) 
% initialize the flag variable
set(handles.genTarget, 'UserData', 0);

% While the flag variable is zero, the loop continues
while( get( handles.genTarget, 'UserData' ) == 0 )

    circlev2(handles.realTarget,handles.range); % print target on real target
    placeT( ...);
    handles.btndown = 0;
    placeT( ...);    
    drawnow
end

This is the "Stop" button

 function simStop_Callback(hObject, eventdata, handles)
% toogle the flag variable to stop other processes.
set( handles.genTarget, 'UserData', 1);
guidata( hObject, handles );

This is the placeT function

 function  placeT( display, h, tgMx, range , btndown, emp, NoMx)
.....
PPIline = plot(h.radarDisplay, X, Y, 'Color', 'y' );
....
pause(0.01);
delete(PPIline)

I have tried the interrupt method, in which when the Stop button is press, the "handles.genTarget" is set to "1" and it will cause the yellow needle stop moving, but it did not work.

Was it due to my interrupt method is wrong? Or I have used the wrong way to delete the line?

Any advice is much appreciated. Thanks and Happy New Year.