Deep Learnign pre-processing image datastore

Good morning,

I'm trying to simplify my question which I'm not able to solve. I've got an image datastore of grayscale imges with size [1674 1396]. I have a function which basically extract a ROI from the image and the resize it to fit the input layer of the net and also convert to rgb. The function i wrote is the following:

function M = myFunc(I) %image as input; image as output
    I = imadjust(I);
    BW = false(size(I,1),size(I,2)); % Create empty mask.
    % Draw ROIs
    xPos = [133.3982 236.8818 1131.7109 1239.2527 1271.7182 1156.0600 261.2309 163.8345];
    yPos = [236.8891 113.1145 102.9691 242.9764 1452.3145 1588.2636 1600.4382 1492.8964];
    m = size(BW, 1);
    n = size(BW, 2);
    addedRegion = poly2mask(xPos, yPos, m, n);
    BW = BW | addedRegion;
    % Create masked image.
    M = I;
    M(~BW) = 0;
    M = imresize(M,[224 224]);
end

According to this example https://it.mathworks.com/help/deeplearning/ug/datastores-for-deep-learning.html at the paragraph Example: Transform Image Datastore to Train Digit Classification Network

I optain the following error:

Error using trainNetwork (line 170)
Invalid training data. Responses must be nonempty.

like in this topic

https://it.mathworks.com/matlabcentral/answers/588652-invalid-training-data-responses-must-be-nonempty-when-using-networktrain-on-cnn

but I'm not able to fix my function to perform my task.

Here my full script:

imPath = 'C:\Users\zanoli.TECNO06\Documents\MATLAB\FILM\Pictures\'; %path to images
imds = imageDatastore(imPath,'FileExtensions','.jpg',...
    "IncludeSubfolders",true,"LabelSource","foldernames"); %definition of an image-datastore
numClasses = numel(categories(imds.Labels)); % count how many different labels are present

dsnew = transform(imds,@preprocessForTraining,'IncludeInfo',true); %definition of transformed datastore applying transform function defined by "@"

%% standard commands for googlenet
net = googlenet; 
lgraph = layerGraph(net); 
newFc = fullyConnectedLayer(numClasses,"Name","new_fc"); 
lgraph = replaceLayer(lgraph,"loss3-classifier",newFc); 
newOut = classificationLayer("Name","new_out"); 
lgraph = replaceLayer(lgraph,"output",newOut); 
options = trainingOptions("sgdm","InitialLearnRate", 0.01); %
ly = net.Layers;
inputLayer = ly(1);
inputSize = inputLayer.InputSize; 
targetSize = [inputSize(1) inputSize(2)]; 
%% End of standard commands for googlenet


[filmNet,info] = trainNetwork(dsnew, lgraph, options); % do the trainig but error appears here!!!!!!!

function [dataOut, info] = preprocessForTraining(data,info)
data = imadjust(data);
% Create empty mask.
BW = false(size(data,1),size(data,2));
% Draw ROIs
xPos = [133.3982 236.8818 1131.7109 1239.2527 1271.7182 1156.0600 261.2309 163.8345];
yPos = [236.8891 113.1145 102.9691 242.9764 1452.3145 1588.2636 1600.4382 1492.8964];
m = size(BW, 1);
n = size(BW, 2);
addedRegion = poly2mask(xPos, yPos, m, n);
BW = BW | addedRegion;
% Create masked image.
dataOut = data;
dataOut(~BW) = 0;
dataOut = imresize(dataOut,[224 224]);
end

Why I'm not able to perform my task?

Any suggestion is kindly appreciated. Thank you!