Your code creates app.UserData, but the error message tells you, that the output UserData is not created:
function UserData = ReadChkptFile(app, FileSelected, File_ID)
In addition the 2nd and 3rd input argument are marked in the editor, which means, that they are not used anywhere. You call this function like this:
app.UserData = ReadChkptFiole(app, app.FileSelected, app.File_ID);
But most likely this would be sufficient already:
because the variables for input and output are contained in the app already.
By the way, the expression fullfile(file) is exactly the same as file , so simply omit the fullfile() in the definition of app.FileSelected.