All data is null

Sorted Rows in Power Query will not affect sorting of data in Visuals. The ONLY reason to have Sorted Rows in Power Query is if the sorting is needed for something like a Row Number column or Running Total, where the order is important. In your charts and graphs, you always have the ability to sort by some column in the visual.

Try removing the Sort operation to make your query go faster.

Now to your issue, try filtering the data to remove NULLs. I bet this is Excel data, which is infamous for having extra rows and columns that nobody wants.