I also get this behavior, crash (SIGILL) when I Unpair from Xcode Devices & Simulators. And NOW, I can only directly connect via USB-C ... however I cannot wirelessly pair. Error "Pairing Failed (-402653147)" (0x17FFFFDB) in Apple Configurator 2's Pairing feature.
Reproduction:
Apple TV connected via USB-C
I was initially able to connect using the network. I was able to Develop with Xcode through Wifi on my mac 11.2.2 - it was beautiful.
Then I THOUGHT I was having problems - turned out I was on the wrong Wifi SSID - so I Unpaired my Apple TV HD to diagnose.
BAM. Then Xcode 12.4 (17801) crashed. "-[__NSCFString appendString:]: nil argument"
I realized my SSID mistake, rejoined the correct Wifi network, and I tried to launch Xcode again and re-pair, but I am no longer able to make that small "Connect via network" check-box become checked, anymore.
I try Connecting via USB-C, see the device, click Unpair, and BAM same crash.
I've tried wiping / resetting the Apple TV. And removing the user. And of course rebooting it.
I have not tried restarting my MAC. I wonder if my mac's Xcode preferences are in a bad state due to the crash?
LOG when Xcode crashes:
Crashing on exception: -[__NSCFString appendString:]: nil argument
Application Specific Backtrace 1:
0 CoreFoundation 0x00007fff206186af __exceptionPreprocess + 242
1 DVTFoundation 0x00000001085b9865 DVTFailureHintExceptionPreprocessor + 424
2 libobjc.A.dylib 0x00007fff203503c9 objc_exception_throw + 48
3 CoreFoundation 0x00007fff206cca9a -[__NSCFString characterAtIndex:].cold.1 + 0
4 CoreFoundation 0x00007fff2065aa15 mutateError + 121
5 CoreFoundation 0x00007fff20581a43 -[__NSCFString appendString:] + 69
6 IDEFoundation 0x0000000109076753 -[IDERunDestinationWithNameAddition _nameWithAddition] + 124
7 IDEFoundation 0x0000000109076ad1 -[IDERunDestinationWithNameAddition _baseName] + 444
8 IDEFoundation 0x0000000109076dd5 -[IDERunDestinationWithNameAddition fullDisplayName] + 58
9 IDEKit 0x000000010a481afa __33-[IDEKeyDrivenNavigableItem name]_block_invoke + 96
10 IDEKit 0x000000010a481411 -[IDEKeyDrivenNavigableItem cachedValueForProperty:withCreationBlock:] + 94
11 IDEKit 0x000000010a481a2b -[IDEKeyDrivenNavigableItem name] + 90
12 DVTKit 0x0000000108d63e70 -[DVTPathCell _delegate_titleForPathComponentCell:atIndex:] + 87
13 DVTKit 0x0000000108d64215 __40-[DVTPathCell _componentCellsFromItems:]_block_invoke + 210
14 CoreFoundation 0x00007fff20592b87 __NSARRAY_IS_CALLING_OUT_TO_A_BLOCK__ + 7
15 CoreFoundation 0x00007fff2062bd0b -[__NSArrayI_Transfer enumerateObjectsWithOptions:usingBlock:] + 161
16 DVTKit 0x0000000108d64053 -[DVTPathCell _componentCellsFromItems:] + 181
17 DVTKit 0x0000000108d64a08 -[DVTPathCell _componentCellsForCurrentlyObservedItems] + 198
18 DVTKit 0x0000000108d64aac -[DVTPathCell sizeWantedForFrame:inView:] + 89
19 DVTKit 0x0000000108c750db -[DVTPathControl sizeWanted] + 253
20 IDEKit 0x000000010a561ab0 -[IDESchemeToolbarController _updateSize] + 100
21 IDEKit 0x000000010a560945 __38-[IDESchemeToolbarController loadView]_block_invoke.160 + 34
22 IDEKit 0x000000010a562383 -[IDESchemeToolbarSegmentedControlView updateConstraints] + 119
23 AppKit 0x00007fff22e6c45f -[NSViewController updateViewConstraints] + 251
24 AppKit 0x00007fff22e674af -[NSView _updateConstraintsForSubtreeIfNeededCollectingViewsWithInvalidBaselines:] + 726
25 AppKit 0x00007fff22e7731f __82-[NSView _updateConstraintsForSubtreeIfNeededCollectingViewsWithInvalidBaselines:]_block_invoke + 270
26 CoreAutoLayout 0x00007fff272f03fa -[NSISEngine withBehaviors:performModifications:] + 84
27 AppKit 0x00007fff22e6729f -[NSView _updateConstraintsForSubtreeIfNeededCollectingViewsWithInvalidBaselines:] + 198
28 AppKit 0x00007fff22e7731f __82-[NSView _updateConstraintsForSubtreeIfNeededCollectingViewsWithInvalidBaselines:]_block_invoke + 270
...
Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 com.apple.AppKit 0x00007fff23168ec8 -[NSApplication _crashOnException:] + 347
1 com.apple.AppKit 0x00007fff22f41041 __62+[CATransaction(NSCATransaction) NS_setFlushesWithDisplayLink]_block_invoke + 828
...