why can't i get the value from the TEXTEDITOR

why can't i get the value from the TEXTEDITOR

coding as below:

CALL METHOD texteditor->get_line_text

EXPORTING

line_number = 12

IMPORTING

text = text_line.

if the value will be geted in the variable text_line.?

but i tried, it has no effect``so can somebody tell me why?