Backend process isn't killed when app is closed (rare)

[petargyurov]

Describe the bug
On a couple of occasions I have observed that if you initiate a child process on the backend and close the app, the child process is not killed.

To Reproduce
(Hard to replicate!) Steps to reproduce the behavior:

1. Go to Detection page
2. Start the detection process
3. Wait for image processing to begin
4. Close app
5. Observe that data processing finishes eventually

Expected behavior
All backend processes to stop when the app is closed.

Screenshots
N/A
System Information (please complete the following information):

• OS: Windows
• Version 10 (1909)

Additional context
Hard to replicate. Willing to close if I cannot replicate enough times.