EXC_BAD_ACCESS (code=2) when NSTextField in NSOutlineView has a connected IBOutlet action

I loaded your app and ran with XCode 12GM / MacOS 10.15.6.

It crashes systematically.

I once succeeded to run with XCode 11.6, and it seemed to work OK when you quit with Quit but crash after an XCode Stop.

• I added in AppDelegate:

Code Block 
func applicationDidFinishLaunching(_ notification: Notification) {
      print("launched")
}

• And it worked.

• I commented applicationDidFinishLaunching out, then app launched but crashed after a few seconds

• I repeated several times (uncommenting applicationDidFinishLaunching, then commenting out again), always the same.

• If I comment out only the print("launched"), it crashes as well.

I changed the value from 5 to 3 in:

Code Block 
    func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int {
        return item == nil ? 5 : 0
    }

And it crashed.
But after disabling IBAction, does not crash anymore.

So I disabled IBAction and removed didFinishLaunching:
No crash effectively.

So, there may be some memory management issue, like zombie. Requires more investigation.

Note I once got the following error log (but not causing crash):

2020-09-25 19:24:19.778052+0200 problem[53542:4126339] [Layout] Unable to simultaneously satisfy constraints:
(
"<NSLayoutConstraint:0x60000212c0a0 V:|-(5)-[NSTextField:0x101817f30] (active, names: filename:0x101818270, '|':filename:0x101818270 )>",
"<NSLayoutConstraint:0x60000212c0f0 V:[NSTextField:0x101817f30]-(57)-| (active, names: filename:0x101818270, '|':filename:0x101818270 )>",
"<NSLayoutConstraint:0x60000212c550 'NSView-Encapsulated-Layout-Height' filename.height == 17 (active, names: filename:0x101818270 )>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x60000212c0f0 V:[NSTextField:0x101817f30]-(57)-| (active, names: filename:0x101818270, '|':filename:0x101818270 )>

Set the NSUserDefault NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints to YES to have -[NSWindow visualizeConstraints:] automatically called when this happens. And/or, set a symbolic breakpoint on LAYOUTCONSTRAINTSNOTSATISFIABLE to catch this in the debugger.
2020-09-25 19:24:19.779542+0200 problem[53542:4126339] [Layout] Unable to simultaneously satisfy constraints:
(
"<NSLayoutConstraint:0x60000212ca00 V:|-(5)-[NSTextField:0x10181d020] (active, names: filename:0x10181d360, '|':filename:0x10181d360 )>",
"<NSLayoutConstraint:0x60000212ca50 V:[NSTextField:0x10181d020]-(57)-| (active, names: filename:0x10181d360, '|':filename:0x10181d360 )>",
"<NSLayoutConstraint:0x60000212cd70 'NSView-Encapsulated-Layout-Height' filename.height == 17 (active, names: filename:0x10181d360 )>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x60000212ca50 V:[NSTextField:0x10181d020]-(57)-| (active, names: filename:0x10181d360, '|':filename:0x10181d360 )>

Set the NSUserDefault NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints to YES to have -[NSWindow visualizeConstraints:] automatically called when this happens. And/or, set a symbolic breakpoint on LAYOUTCONSTRAINTSNOTSATISFIABLE to catch this in the debugger.
2020-09-25 19:24:19.780800+0200 problem[53542:4126339] [Layout] Unable to simultaneously satisfy constraints:
(
"<NSLayoutConstraint:0x60000212d1d0 V:|-(5)-[NSTextField:0x101821040] (active, names: filename:0x101821380, '|':filename:0x101821380 )>",
"<NSLayoutConstraint:0x60000212d220 V:[NSTextField:0x101821040]-(57)-| (active, names: filename:0x101821380, '|':filename:0x101821380 )>",
"<NSLayoutConstraint:0x60000212d540 'NSView-Encapsulated-Layout-Height' filename.height == 17 (active, names: filename:0x101821380 )>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x60000212d220 V:[NSTextField:0x101821040]-(57)-| (active, names: filename:0x101821380, '|':filename:0x101821380 )>

Set the NSUserDefault NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints to YES to have -[NSWindow visualizeConstraints:] automatically called when this happens. And/or, set a symbolic breakpoint on LAYOUTCONSTRAINTSNOTSATISFIABLE to catch this in the debugger.
2020-09-25 19:24:19.781907+0200 problem[53542:4126339] [Layout] Unable to simultaneously satisfy constraints:
(
"<NSLayoutConstraint:0x60000212d9a0 V:|-(5)-[NSTextField:0x101823c50] (active, names: filename:0x101823f90, '|':filename:0x101823f90 )>",
"<NSLayoutConstraint:0x60000212d9f0 V:[NSTextField:0x101823c50]-(57)-| (active, names: filename:0x101823f90, '|':filename:0x101823f90 )>",
"<NSLayoutConstraint:0x60000212dd10 'NSView-Encapsulated-Layout-Height' filename.height == 17 (active, names: filename:0x101823f90 )>"
)

Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x60000212d9f0 V:[NSTextField:0x101823c50]-(57)-| (active, names: filename:0x101823f90, '|':filename:0x101823f90 )>

Set the NSUserDefault NSConstraintBasedLayoutVisualizeMutuallyExclusiveConstraints to YES to have -[NSWindow visualizeConstraints:] automatically called when this happens. And/or, set a symbolic breakpoint on LAYOUTCONSTRAINTSNOTSATISFIABLE to catch this in the debugger.

Thanks for your reply. I already tried investigating with zombies turned on and even using Instruments, but they were of no help.

So, for the time being, may add appDidFinishLauncing to your app.
And file a bug report with the sample project (don't forget to attach all the tests that were done).

Good luck.

this one issue is driving me crazy ~ tried investigating 

You might try burning a support ticket w/DTS via the Member Center to see if they can shed light - they will need a buildable project that demos the issue...

Good luck.