The assistant was designed to take what was last written in a note area and use the text to assist the user. For example, you can write "CALL STEVE" in the notepad and tap assist without highlighting the text. This makes Newton's notepad acts like a slip of paper where you can jot down an instruction and say "do it."
You should tap once somewhere in a blank space area on the notepad and then tap assist if you want the assistant to come up without text in the assistant window.