Detect FAST feature from whole Training dataset. getting Error Struct contents reference from a non-struct array object.

if true
 numImages = numel(trainingSet1.Files);
for i = 1:numImages
  img = readimage(trainingSet1, i);

    img = rgb2gray(img);

    % Apply pre-processing steps
    img = imbinarize(img);

    trainingFeatures(i) = detectFASTFeatures(img);    
    end
  end

Error in this line

if true
      trainingFeatures(i) = detectFASTFeatures(img);   
end

error is this

if true
 Error using fast (line 18)
 Struct contents reference from a non-struct array object.
end