Real Time Subplot Error

Hello,

I would like to create a realtime subplot with Matlab.

I get data from four different channels.

I have written the following code for it:

%%
Vref = 2.42;
n = 24;
gain = 12;
LSB = (2*Vref)/gain/((2^(24))-1);


%figure
figure('Renderer', 'painters', 'Position', [10 10 2000 500])
h1 = animatedline('Color','r','LineWidth',1);
h2 = animatedline('Color','r','LineWidth',1);
h3 = animatedline('Color','r','LineWidth',1);
h4 = animatedline('Color','r','LineWidth',1);
ax = gca;
ax.YGrid = 'on';

stop = false;
startTime = datetime('now');
while ~stop
    % Read current voltage value
    fprintf(bt, 'y');
    Val1 = fscanf(bt, '%f');
    Vout1 = Val1*LSB;
    pause(5/100000);
    fprintf(bt, 'c');
    Val2 = fscanf(bt, '%f');
    Vout2 = Val2*LSB;
    pause(5/100000);
    fprintf(bt, 'v');
    Val3 = fscanf(bt, '%f');
    Vout3 = Val3*LSB;
    pause(5/100000);
    fprintf(bt, 'b');
    Val4 = fscanf(bt, '%f');
    Vout4 = Val4*LSB;
  
    subplot(4,1,1)

    % Get current time
    t =  datetime('now') - startTime;
    % Add points to animation
    addpoints(h1,datenum(t), Vout1)
    % Update axes
    %ylim([-2 2])
    ax.XLim = datenum([t-seconds(15) t]);
    datetick('x','keeplimits')
    drawnow
    
    subplot(4,1,2)
 
    % Get current time
    t =  datetime('now') - startTime;
    % Add points to animation
    addpoints(h2,datenum(t), Vout2)
    % Update axes
    %ylim([-2 2])
       ax.XLim = datenum([t-seconds(15) t]);
    datetick('x','keeplimits')
    drawnow
    
    subplot(4,1,3)

    % Get current time
    t =  datetime('now') - startTime;
    % Add points to animation
    addpoints(h3,datenum(t), Vout3)
    % Update axes
    %ylim([-2 2])
    ax.XLim = datenum([t-seconds(15) t]);
    datetick('x','keeplimits')
    drawnow
    
    subplot(4,1,4)

    % Get current time
    t =  datetime('now') - startTime;
    % Add points to animation
    addpoints(h4,datenum(t), Vout4)
    % Update axes
    %ylim([-2 2])
    ax.XLim = datenum([t-seconds(15) t]);
    datetick('x','keeplimits')
    drawnow
    

end

Unfortunately I always get the error message:

"Invalid or deleted object.

Error in readbluetooth2 (line 208)

ax.XLim = datenum([t-seconds(15) t]);"

What am I doing wrong?