Crash when clicking the dock icon when the main window is closed

**Describe the issue**
When a PowerPC mac99 virtual machine was selected in the main window, and after having closed the main virtual machine list window (with a VM running, so UTM does not exit), clicking the dock icon will crash UTM immediately. If an x86 or ARM virtual machine was selected in the list, the main window will reopen without issue. It does not appear to matter which virtual machine is running, only which was selected in the list.

**Configuration**
* UTM Version: 2.1.1 (30)
* OS Version: 11.4 (20F71)
* Intel or Apple Silicon? Apple Silicon

**Crash log**
[UTM_2021-06-21-115434_Motoko.crash.txt](https://github.com/utmapp/UTM/files/6688871/UTM_2021-06-21-115434_Motoko.crash.txt)

**Debug log**
[debug.log](https://github.com/utmapp/UTM/files/6688884/debug.log)

**Upload VM**
[config.plist.txt](https://github.com/utmapp/UTM/files/6688879/config.plist.txt)