Emoji viewer constantly popping up in input fields
I faced the same annoying bug with emoji viewer in macOS Mojave (10.14.6). 

Possible solution

In case of multiple input sources, try to delete one of them (System Preferences -> Keyboard -> Input sources) and add deleted one again. System restart was not required in my case.

If you have only one default input source, try to add some another and then delete it.

I hope this solution comes in handy