I know, I doesn't make any sense.
However I remember that in a thread another forum member had a problem where he had to delete and rewrite a value on TacText and then it worked.
http://ffhacktics.com/smf/index.php?topic=10676.msg202372#msg202372
So maybe the value that your null string might be a valid value for TacText when using the Edit Selection add-in, but no when deleting the cell contents from FFText sheet.
EDIT: I just added a bunch of new stuff and it keeps working.
However I remember that in a thread another forum member had a problem where he had to delete and rewrite a value on TacText and then it worked.
http://ffhacktics.com/smf/index.php?topic=10676.msg202372#msg202372
So maybe the value that your null string might be a valid value for TacText when using the Edit Selection add-in, but no when deleting the cell contents from FFText sheet.
EDIT: I just added a bunch of new stuff and it keeps working.