IOM crash: observable::~Observable() - 1 observers still watching, first: 20TransformGradientBox

[humdinger]

I happened to trigger this crash twice, but cannot reliably reproduce it at will. Both times it happened after changing a style from Gradient to Color. It may have something to do with doing an Undo before changing the style type...

Full report attached.

thread 55735: w>Icon-O-Matic 
state: Call (Observable::~Observable() - 1 observers still watching, first: 20TransformGradientBox
)
Frame		IP			Function Name
-----------------------------------------------
00000000	0x10561e460a7	_kern_debugger + 0x7 
	Disassembly:
		_kern_debugger:
		0x0000010561e460a0:   48c7c0e5000000  mov $0xe5, %rax
		0x0000010561e460a7:             0f05  syscall  <--

0x7f3a355cf750	0x1d29783fe42	Observable::~Observable() + 0x82 
0x7f3a355cf770	0x1d29781c023	Gradient::~Gradient() + 0x63 
0x7f3a355cf790	0x1d29781c0b1	non-virtual thunk to Gradient::~Gradient() + 0x11 
0x7f3a355cf7c0	0x1b499b371a8	BReferenceable::ReleaseReference() + 0x78 
0x7f3a355cf870	0x1d297857723	StyleView::_SetGradient(Gradient*, bool, bool) + 0x113 
0x7f3a355cf8a0	0x1d297857956	StyleView::ObjectChanged(Observable const*) + 0x156 
0x7f3a355cf900	0x1d29783fef3	Observable::Notify() const + 0x63 
0x7f3a355cf940	0x1d297873bee	SetGradientCommand::Perform() + 0xfe 
0x7f3a355cf970	0x1d29782ac6e	CommandStack::Perform(Command*) + 0x3e 
0x7f3a355cf9a0	0x1d297857e36	StyleView::MessageReceived(BMessage*) + 0xa6 
0x7f3a355cfa30	0x1b499ad718b	BWindow::task_looper() + 0x1cb 
0x7f3a355cfa50	0x1b499a1416d	BLooper::_task0_(void*) + 0x1d 
0x7f3a355cfa70	0x10561e44e16	thread_entry + 0x16 
00000000	0x7f4abd89b258	commpage_thread_exit + 0