That's not supposed to happen. I'm guessing that maybe it happens when you've left the app running in the background and maybe your phone aggressively kills off the connection to save battery or something like that.
It would help if you could do a little experimentation to see when it's happening ... e.g. kill the app whenever you exit, and see if it will happen even when the app is starting from scratch to confirm or disprove the above theory.
Also, after it happens, you can use "Report Bug" from the profile pic dropdown (click on your profile pic) and check the checkbox to include browser logs so we can see if they'll check any light on what's happening. (If is the phone shutting down the connection, the logs may not help much though -- but just worth a check anyway.)
We don't really want people to be seeing the error and will see what we can do to avoid it ... but as you seem to be confirming, it is probably harmless -- i.e. it should reconnect and work properly if it's due to the connection having been shut down for some reason.