Excuse me for not providing the answer. I just want to confirm that the issue is really happening.

Today, I had a meeting with my client. I noted important content in TextEditor and I haven't saved it yet. After the meeting, I did several zoom-in and zoom-out. And suddenly, TextEditor became not responding and it has been eating more and more RAM (current is 23.14 GB).

I would thank you a lot if anyone tells me how can I restore/save the currently opening content. If I force killing the process, I think everything will be lost.

Oh my god, Apple. Please don't do this to me! :'(