I suspect all that is contained in...
~/Library/Containers/com.apple.iChat
~ tilde indicates hidden Home Library.
Apple hid the Users' Library folders...
Method 1:
1 From the Finder, select the Go menu at top of the screen, and choose Go to Folder.
2 In the window that opens, enter ~/Library, and click Go.
Method 2:
1 Go to the Finder (or desktop).
2 Hold the Option key on your keyboard, and click the Go menu at the top of the screen.
3 With the Go menu open, you'll notice that pressing and releasing Option will display or hide the Library choice in this menu.
4 Select Library from the Go menu (while holding down Option) to access the hidden folder.
I'd quit iChat/iMessage/Messages & move that Folder to the Desktop incase we want to put it back, then open the App.