I ran into this error and stuck with it for nearly 2 weeks...

My problem includes two part, first check if any column that is being used as row group field but is null.

Second, click the refresh button at top left in the preview mode.
* I guess that will clear cache like ReportName.rdl.data; But in my case, I can't find any rdl.data file. I guess that's VS version differences. I'm using VS 2017.