Page content replaced with "undefined"

I just delete a photo from a page of content. The entire content of the page disappeared and was replaced with the word “undefined”. I deleted that word and wrote “under construction”, since I now have a blank page that I have to rewrite. When I clicked update to save “under construction” , it reverted to “undefined” again. Does anyone have a solution?
The site is https://grumpyspokanespray.com/noxious-weed-control/

Thanks Jean