Looks like 12.0.1 is now in the Mac App Store to address this very issue.
klm001
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I did the same as Gerrit. My download called itself Version 12.0 (12A7209), just like App Store version, but the documentation reader works properly. Sigh! @TallBassett — did you install the downloaded GM version directly over the App Store version, or did you uninstall the App Store version first?
@josh112 — This seems to be affecting the App Store version only. I had the GM (209) installed from before, and the documentation viewer works fine there. Many of the issues reported in this topic are against the same version, 12A7209, so it might not be related to the delivery method (App Store).
Has anyone tried uninstalling the failing version and reinstalling from the GM instead of from the App Store?
Upgraded on second MacBook Pro to Xcode Version 12.0 (12A7209) from Mac App Store on Catalina 10.15.6 — same issue as on first: Xcode crashes when selecting Developer Documentation from the Window menu. No project needs to be open to reproduce the crash.
Process:							 Xcode [8654]
Path:									/Applications/Xcode.app/Contents/MacOS/Xcode
Identifier:						com.apple.dt.Xcode
Version:							 12.0 (17219)
Build Info:						IDEFrameworks-17219000000000000~2 (12A7209)
App Item ID:					 497799835
App External ID:			 837639917
Code Type:						 X86-64 (Native)
Parent Process:				??? [1]
Responsible:					 Xcode [8654]
User ID:							 501
Date/Time:						 2020-09-17 11:09:25.530 -0400
OS Version:						Mac OS X 10.15.6 (19G2021)
Report Version:				12
Bridge OS Version:		 4.6 (17P6610)
Time Awake Since Boot: 16000 seconds
Time Since Wake:			 5600 seconds
System Integrity Protection: enabled
Crashed Thread:				0	Dispatch queue: com.apple.main-thread
Exception Type:				EXC_CRASH (SIGABRT)
Exception Codes:			 0x0000000000000000, 0x0000000000000000
Exception Note:				EXC_CORPSE_NOTIFY
Application Specific Information:
Sending windowMenu_showDocumentation: to IDEDocCommandHandler from <NSMenuItem: 0x7f98b64de490 Developer Documentation>
ProductBuildVersion: 12A7209
ASSERTION FAILURE in /Library/Caches/com.apple.xbs/Sources/IDEPlugins/IDEPlugins-17192/IDEDocViewer/DocumentationNavigator.swift:142
Details:	
Function: explorableSource(for:)
Thread:	 <NSThread: 0x7f98f55077c0>{number = 1, name = main}
Open FDs: 42/7168
Same issue here running Xcode Version 12.0 (12A7209), updated from Mac App Store on Catalina 10.15.6.