Help with GUI matlab to control a servomotor

Hi, I would like to know if someone could help me to solve this error: "MATLAB connection to Uno at COM3 exists in your workspace. To create a new connection, clear the existing object."

I am working with a slider to move a servomotor and I don´t know how to solve it, I have tried to "clear all" in the last line of the function and it still no working. This is the code:

% --- Executes on slider movement.

function slider1_Callback(hObject, eventdata, handles)

global position

global pos

position=get(handles.slider1,'Value');

pos=double(position);

assignin('base','pos',position);

a = arduino('COM3','Uno','Libraries','Servo');

assignin('base','arduino',a);

s = servo(a,'D2');

assignin('base','s',s);

writePosition(s,pos);

[FRANCISCO JOSÉ DEL MORAL CONDE]

It works once and then it works if I delete the variables "s" and "a" or do a "clear all".