I’m not getting an error, since there is no data for certain channels for certain timebelts, calculation does not apply for those cells. Hence same ‘blank’ cells are apearing even after applying the isNull condition.
Have you tried substituting the null values with the default value using an ‘if’ value is null , substitute with default value condition?
Similar Excerpt from isNull - Amazon QuickSight :
ifelse( isNull({ActiveFlag}) , ‘Inactive’, ‘Active’)
since there is no data for the time belt, I don’t think creating calc filed resolve. I have tagged this as a feature request. Your feedback helps us build a better service.