Flashing not stopped when clicking cancel

[krichardsson]

When flashing a Crazyflie and cancelling the flashing, the actual flashing is not stopped and the radio continues to transmit.

How to reproduce:

1. Open the bootloader dialog box
2. Initiate bootloader cold boot
3. Choose something to flash
4. Click "Program" to start flashing
5. When the programming has started and the progress bar is moving, click "cancel bootloading"

Expected result:
The flashing should stop and the dialog close

Actual result:
Dialog closed but flashing continues which can be seen on the radio LED on the Crazyflie that continues to flash

Cfclient version: 2020.02-105-geb937a1-modified
System: darwin
Python: 3.7.3
Qt: 5.14.1
PyQt: 5.14.1