Get rid of "The Wi-Fi network ... could not be joined." message How can I get rid of this message? 



I've tried clicking both: "Cancel" and "Run Diagnostics" buttons. I've also tried to turn off and back on the Wi-fi. I don't want to restart my mac as there are some important processes running.

I'm using a MacBook Pro (Retina) - macOS High Sierra 10.13.1
Go to Spotlight (magnifying glass in top corner) / Finder > Activity Monitor
Search for SystemUIServer in CPU tab using the search bar located at top right corner 
Force quit it