Crash paying invoice

[Mike Alexander]

Something in "private-kvp" branch that was merged in a day or two ago 
seems to have broken part of the business features.  If I try to pay an 
invoice for a customer I get a crash in the code that creates the 
payment window.  The top of the stack is

* thread #1: tid = 0x81af3a, 0x00000001111f7c32 
libgobject-2.0.0.dylib`g_type_check_instance_cast + 38, queue = 
'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, 
address=0x100000002)

    frame #0: 0x00000001111f7c32 
libgobject-2.0.0.dylib`g_type_check_instance_cast + 38

  * frame #1: 0x00000001139eb92e 
libgncmod-business-gnome.dylib`gnc_payment_dialog_owner_changed(pw=0x00007fe33b2ceff0) 
+ 78 at dialog-payment.c:461

    frame #2: 0x00000001139ed0c6 
libgncmod-business-gnome.dylib`new_payment_window(owner=0x00007fff52546fc0, 
book=0x00007fe33c83c6d0, invoice=0x00007fe33a13a7f0) + 2812 at 
dialog-payment.c:991

    frame #3: 0x00000001139ed504 
libgncmod-business-gnome.dylib`gnc_ui_payment_new_with_invoice(owner=0x00007fe33f26f350, 
book=0x00007fe33c83c6d0, invoice=0x00007fe33a13a7f0) + 110 at 
dialog-payment.c:1101

    frame #4: 0x00000001139e0cd5 
libgncmod-business-gnome.dylib`gnc_invoice_window_payment_cb(widget=0x0000000000000000, 
data=0x00007fe33f26f1f0) + 139 at dialog-invoice.c:1078

    frame #5: 0x00000001139f4f90 
libgncmod-business-gnome.dylib`gnc_plugin_page_invoice_cmd_pay_invoice(action=0x00007fe33a144070, 
plugin_page=0x00007fe33a9004b0) + 294 at gnc-plugin-page-invoice.c:908

I verified that f49983b fails and acad5a0 works.  I tried to do a 
bisect to find the problem, but many of the intermediate commits won't 
build cleanly on my machine so I gave up.  When I reverted that merge 
things started working again.  Hopefully this will be easy to 
reproduce, but if you need more info let me know.

             Mike