The best thing I can do is show you an image.

I reported the same bug and I noticed the same thing that MrDumpster said works.

Can confirm this happens, but it seems to be intermittent. I haven't found the trigger yet, and the only way to fix it is a full /reload.

What caused the bug for me:

Whisper a Player back and forth a couple times
Right click on the Conversation tab and click "close window"
whisper that player again (expectation a new tab opens just like before)
Result: New window opens with no text inside, you can hear whisper ping
perform /reloadui and everything works as advertised