A short answer to your question: Yes, 7.20 is supported.
To give you a pointer to find the cause of your problem.
On https://github.com/mendix/ChartJS you can find a testproject for this widget.
I opened the testproject in 7.20 and the charts rendered as expected.
From the stacktrace it looks like your datasource microflow contains a mapping. It looks like something is going wrong there. Do you use a mapping to set up the data that is being rendered?