MP locks up on scannng comports

build 1.3.8313.6048

when trying to upload firmware to QiotekH743 (and others) MP locks up indefinitely in scanning comports phase...using task manager to kill it leaves a residual process that task manager cannot end, only with a power cycle of the PC....from this point on MP cannot upload firmware on a restart and locks up

seems to be autopilots with an SLCAN virtual port, but I have not definitively tested that this a per-requisite

Start by reproducing the firmware upload failure described for QiotekH743 or another autopilot with an SLCAN virtual port, then observe Mission Planner during the COM-port scanning phase. Investigate the scanning and process-cleanup path, including the residual process after termination. Done means uploads no longer hang and Mission Planner can restart normally without requiring a PC power cycle.

Written by the indexing model from the issue text.