#319 Apply Expression on the last row's blank cells is fatal

[Genreless]

When applying expressions to cells, if you select a column, deselect some members of the column, and then try applying an expression to the column, PFM crashes. This is because PFM tries to apply an expression to the new row at the bottom and gets a null reference. I assume this was coded around for when the entire column is selected since that works, but it is still broken for a partial column selection including the last blank row.