This happened to me on my Mac and this is how I fixed it JUST NOW.
Go to Window > Close
That should close the Messages and bring out the Time Tracker.
You can then open Messages on the bottom right of the tracker.
This is exactly what happend on my mac.
fix: - Quit the Upwork app completly.
- then restart,
-re-open the app.
- then minimize and maximaize.
it worked for me, hope this helps.