You are using widget Datacounter which is causing this error. Try to update it, solve its bug or report it to the maker.
Hm, but DataCounter is not in Mendix's AppStore. Did you create it yourself? And you did state this somewhere:
'innerHTML' in somevariable
whilst somevariable is null, not initialised. That will give you the error you mention.
As Tim mentioned, this error occurs in DataCounter widget.
This error means: You use innerHTML to retrieve the text from a input field or something.
And in the widget you are trying find something using ‘in’ operator with the value returned using innerHTML.
Try to optimize the Datacounter widget with validation checks for the value returned using innerHTML