Xcode 12 quit unexpectedly in Mac OS 10.15.4 - ios

Question

Xcode 12 quit unexpectedly in Mac OS 10.15.4.
The document “Main.storyboard” had an issue that was found and repaired.
This may be due to an SCM operation such as merging. Please save the document to fix the issue.
Multiple resources have the same name: groupTableViewBackgroundColor.
Date/Time: 2020-09-26 08:18:54 +0530
End time: 2020-09-26 08:26:49 +0530
OS Version: Mac OS X 10.15.4 (Build 19E266)
Architecture: x86_64h
Report Version: 29
Data Source: Stackshots
Shared Cache: 0x2fb4000 01EE95E0-91B0-354A-BD0A-C761305CD75D
Command: Xcode
Path: /Applications/Xcode 12.app/Contents/MacOS/Xcode
Identifier: com.apple.dt.Xcode
Version: 12.0 (17219)
Build Version: 2
Product Build Version: 12A7209
Project Name: IDEFrameworks
Source Version: 17219000000000000
Parent: launchd [1]
PID: 2432
Event: hang
Duration: 475.00s
Duration Sampled: 1.99s (process was unresponsive for 473 seconds before sampling)
Steps: 20 (100ms sampling interval)
Hardware model: MacBookAir7,2
Active cpus: 4
Time Awake Since Boot: 5700s
Fan speed: 1234 rpm
--------------------------------------------------
Timeline format: stacks are sorted chronologically
Use -i and -heavy to re-report with count sorting
--------------------------------------------------
Heaviest stack for the main thread of the target process:
20 <truncated backtrace>
20 __psynch_mutexwait + 10 (libsystem_kernel.dylib + 12386) [0x7fff6a22a062]
*20 psynch_mtxcontinue + 0 (pthread + 9566) [0xffffff7f82d6e55e]
Process: Xcode [2432]
UUID: BEF84410-992D-3871-AD2A-C8C9AB4BD25C
Path: /Applications/Xcode 12.app/Contents/MacOS/Xcode
Architecture: x86_64
Parent: launchd [1]
UID: 501
Sudden Term: Tracked
Footprint: 490.54 MB
Start time: 2020-09-26 08:26:47 +0530
End time: 2020-09-26 08:26:48 +0530
Num samples: 20 (1-20)
CPU Time: 0.022s (52.9M cycles, 37.5M instructions, 1.41c/i)
Note: Unresponsive for 473 seconds before sampling
Note: 4 idle work queue threads omitted

Answer 1

I had exactly the same problem.
Good news, I found the solution!
Short story:
You need to find the Views that have tableCellGroupedBackgroundColor as their color and change it.
tip: This color was used by the tableView cells as a background color, look there first.
Long story:
After updating to Xcode 12.2 every time I opened Main.storyboard, the same message was displayed as yours.
Assuming it is a color incompatibility (tableCellGroupedBackgroundColor in our case), I opened Main.storyboard as Source Code (right click on the Main.storyboard -> Open As -> Source Code) and searched for "tableCellGroupedBackgroundColor".
I noticed that this color appeared several times in the "resources" at the end of the file, while it should appear only once.
This is probably why Xcode displays the error message.
Looking above, I noticed that the specific color was in the background color in tableView cells.
So I reopened Main.storyboard as Interface Builder (right click on the Main.storyboard -> Open As -> Interface builder-Storyboard) and changed the background color in all tableView cells to Default.
pro tip: You can delete the colors from the Source code directly, by deleting the lines that refers the colors to the Views.
That's it!
After that everything works perfectly again!

Answer 2

I have a similar issue with additional infos. In xCode 12, and 12.2 beta, my main storyboard appear after many minutes and I get:
The document “Main.storyboard” had 15 issues that were found and repaired.
The save popup also shows:
This may be due to an SCM operation such as merging. Please save the document to fix the issues.
The errors are of 3 types for me:
1- Multiple resources have the same name: darkTextColor.
2- Multiple resources have the same name: groupTableViewBackgroundColor.
3- Multiple resources have the same name: tableCellGroupedBackgroundColor.
It asks to save in order to repair but it does not repair at all.
There are no warning in the environment either before or after the save.
When the project finally loads, if I edit it for a while then it stops responding and I have to force xCode to quit. If I load my app in xCode 11.7 it shows instantly and work fine. If one of our users upgrades to IOS14 with the working version of the app it crashes in some views however if I compile with xCode12, even with storyboard file that I cannot open, and I publish on IOS14 the apps work well and the broken views shows up.
When I debug in xCode 12 and navigate to a broken view the app/xCode stops but no warning or error are displayed. I tried to change and remove this line in the storyboard file without success: