"Object reference not set to an instance to an object" when try to group the table in Power Query

In the past I have seen this error message appear when there is an issue with the file, most often the file cannot find the data model.  Please try save-as with a different file name, close and re-open the file and see if that resolves the issue.

I had this issue , thing i did to resolve was checking null values,variables or expressions in the query, make sure that they are not null or empty. You can do this by adding a check for null values in the query or by using the "IsBlank" function in Power BI.