Crash if screenlock/sleep activates while downloading Apple VM

**Describe the issue**

The app was slowly downloading an IPSW for a new macOS VM (🎉 btw!) when I walked away to use another machine. When I came back and unlocked the machine, UTM was no longer running and I got the attached crash report.

**Configuration**
* UTM Version: Version 3.0.0 (41)
* OS Version: macOS 12.1 (21C52)
* Intel or Apple Silicon? Apple M1 Pro

**Crash log**

Attached.

[UTM-2022-01-06-173810.ips.txt](https://github.com/utmapp/UTM/files/7825749/UTM-2022-01-06-173810.ips.txt)

**Debug log**

The placeholder VM entry that was showing the download progress has disappeared, so it's unclear if/how to get UTM-specific logs. I did find a separate "diagnostic report" which I'm including in the attachments.

[UTM_2022-01-06-173026_wilson.diag.txt](https://github.com/utmapp/UTM/files/7825750/UTM_2022-01-06-173026_wilson.diag.txt)

**Upload VM**

Was basically just walking through the wizard to create a macOS VM.

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Additional notes: I do have an application firewall (Little Snitch) which may have timed out my connection approval and blocked a download socket(s) or something. Trying again this time with a 2hr approval in case it makes a difference. [This might be particularly relevant given the `com.apple.NSURLSession-delegate` thread shown as the culprit!]

Also, while the download was happening I happened to delete the VM right above the "placeholder" one. This caused the UI to select the downloading one which is normally not possible. I forget what exactly the window showed because I simply clicked away and moved on at the time. This crash doesn't look particularly UI-related at first glance but mentioning this as well, just in case.