[WebPositive] crash in BMessenger::operator==(BMessenger const&)

[diver]

Happened after closing Web+ (using Alt+q) during app startup.

Debug information for team /boot/system/apps/WebPositive (788):
CPU(s): 1x Intel Core™ i7-3635QM
Memory: 998.94 MiB total, 360.09 MiB used
Haiku revision: hrev54154+31 May 13 2020 03:10: (x86_64)

Active Threads:
	thread 788: WebPositive (main)
	thread 822: w>WebPositive: Save 
	thread 826: team 788 debug task 
	thread 824: w>WebPositive 
		state: Exception (Segment violation)

		Frame		IP			Function Name
		-----------------------------------------------
		0x7f35aeed7b70	0xfdc824dd54	BMessenger::operator==(BMessenger const&) const + 0x4 
			Disassembly:
				BMessenger::operator==(BMessenger const&) const:
				0x000000fdc824dd50:             31c0  xor %eax, %eax
				0x000000fdc824dd52:             8b16  mov (%rsi), %edx
				0x000000fdc824dd54:             3917  cmp %edx, (%rdi) <--

			Frame memory:
				[0x7f35aeed7b68]  Xwq.....   58 77 71 80 8d 01 00 00
		0x7f35aeed7bc0	0x18d80717753	SettingsMessage::RemoveListener(BMessenger const&) + 0x63 
		0x7f35aeed7c10	0x18d8071f65e	BrowserWindow::~BrowserWindow() + 0x3e 
		0x7f35aeed7c30	0x18d8071f6dc	BrowserWindow::~BrowserWindow() + 0xc 
		0x7f35aeed7c50	0xfdc82423bc	BLooper::_task0_(void*) + 0x2c 
		0x7f35aeed7c70	0x1e2f45f5f67	thread_entry + 0x17 
		00000000	0x7f46e880f260	commpage_thread_exit + 0

[cocobean]

diver - Tested with Haiku R1B3 (+56) x64 and Web+ 1.2-alpha/HaikuWebKit 1.8.2-2. Open and closed (Alt+Q) Web+ four times consistently without any crash. No crash.