Non-existent user defined runtime attribute - objective-c

Question

I have a simple Mac application that receives the following error when opening a document:
Failed to set (allowsDragging) user defined inspected property on (PKMPDFView): [ setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key allowsDragging.
If I intercept the "set" attempt, I see that it originates from the method -[NSIBUserDefinedRuntimeAttributesConnector establishConnection] during nib loading.
Full trace:
0 PDFApp 0x0000000100003592 -[PKMPDFView setValue:forUndefinedKey:] + 49
1 Foundation 0x00007fff309b8dde -[NSObject(NSKeyValueCoding) setValue:forKey:] + 331
2 AppKit 0x00007fff2c08af15 -[NSView setValue:forKeyPath:] + 394
3 AppKit 0x00007fff2c00d029 -[NSIBUserDefinedRuntimeAttributesConnector establishConnection] + 637
4 AppKit 0x00007fff2bdd9719 -[NSIBObjectData nibInstantiateWithOwner:options:topLevelObjects:] + 1430
5 AppKit 0x00007fff2bdd0991 loadNib + 435
6 AppKit 0x00007fff2bdcfeb5 +[NSBundle(NSNibLoading) _loadNibFile:nameTable:options:withZone:ownerBundle:] + 696
7 AppKit 0x00007fff2c005aba +[NSBundle(NSNibLoadingInternal) _loadNibFile:externalNameTable:options:withZone:] + 150
8 AppKit 0x00007fff2c005893 -[NSWindowController loadWindow] + 322
9 AppKit 0x00007fff2be18cf5 -[NSWindowController window] + 84
10 AppKit 0x00007fff2bfd8134 -[NSWindowController showWindow:] + 36
11 AppKit 0x00007fff2be18c7a -[NSDocument showWindows] + 116
12 PDFApp 0x0000000100002e75 -[PKMPDFDocument showWindows] + 50
13 AppKit 0x00007fff2c35f13e __80-[NSDocumentController openDocumentWithContentsOfURL:display:completionHandler:]_block_invoke.1039 + 182
14 AppKit 0x00007fff2c36ef70 ___NSMainRunLoopPerformBlock_block_invoke + 25
15 CoreFoundation 0x00007fff2e897a3c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
16 CoreFoundation 0x00007fff2e87a973 __CFRunLoopDoBlocks + 275
17 CoreFoundation 0x00007fff2e879ffe __CFRunLoopRun + 1278
18 CoreFoundation 0x00007fff2e879867 CFRunLoopRunSpecific + 487
19 HIToolbox 0x00007fff2db59d96 RunCurrentEventLoopInMode + 286
20 HIToolbox 0x00007fff2db59b06 ReceiveNextEventCommon + 613
21 HIToolbox 0x00007fff2db59884 _BlockUntilNextEventMatchingListInModeWithFilter + 64
22 AppKit 0x00007fff2be09a73 _DPSNextEvent + 2085
23 AppKit 0x00007fff2c59fe34 -[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
24 AppKit 0x00007fff2bdfe885 -[NSApplication run] + 764
25 AppKit 0x00007fff2bdcda72 NSApplicationMain + 804
26 libdyld.dylib 0x00007fff56848015 start + 1
What confuses me is that PKMPDFView is a direct subclass of NSView and has no user defined properties associated with it in the .xib file being loaded. This view also has no outlets defined in the .xib, just a referencing outlet (contentView) from its document. Running find/grep through my development directory finds no instance of "allowsDragging" anywhere. Likewise, using the strings utility reveals no "allowsDragging" in the built app.
I've tried all the standard project cleaning with no change in symptoms.
At the moment, I'm tempted to simply override setValue:forUndefinedKey: and forget it but I'm still curious. Does anyone have information or good guesses about where this would be coming from?
(This is Xcode 10.1 on macOS 10.13.6, in case that matters.)