Semi-rapid tab switching results in strange connection state

[microbit-robert]

If you have a micro:bit successfully connected with the editor showing the serial, switch to a different tab briefly (maybe about half a second) and then switch back again. On returning to the editor, the serial is gone. Note that the program does not reset in this case (this is prior to #1103 being merged). The serial returns if you flash the device, or switch tabs in the same manner again.

Discovered while testing #1103, but not related to any change therein as it occurs on the production deployment.