assertion in fontcache in qtwebkit

ASSERTION FAILED: fontCache()->generation() == m_generation
platform\graphics\FontGlyphs.cpp(109) : WebCore::FontGlyphs::realizeFontDataAt
1 5F596327
2 5EA14610
3 5E7E8D65
4 5E7E8DB8
5 5E7E8D40
6 5E7D8BDF
7 5E97AF5C
8 5E97B0B2
9 5E6DDD79
10 5E6EAA76
11 5E6DEB26
12 5E6E47D9
13 5E6FAAA8
14 5E6EA45F
15 5E6DEB39
16 5E6E47D9
17 5E71568F
18 5E712835
19 5E6276B3
20 5E62B82E
21 5E159CA7
22 67095EB6
23 67095E74
24 670A2B68
25 6658F27C
26 670A275C
27 6654471E
28 665428D1
29 65744597
30 65857C98
31 665962BF
(44c.910): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.

• • • WARNING: Unable to verify checksum for C:\Users\Administrator\repo\dbuild\temp_folder\Sigil\Qt5WebKitd.dll
      eax=0000001f ebx=00000000 ecx=bed69407 edx=00000004 esi=059092a8 edi=002e7ad8
      eip=5f596327 esp=002e5ae4 ebp=002e5ae4 iopl=0 nv up ei pl zr na pe nc
      cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00210246
      Qt5WebKitd!WTFCrash+0x17:
      5f596327 c705efbeadbb00000000 mov dword ptr ds:[0BBADBEEFh],0 ds:0023:bbadbeef=????????

The complete backtrace from a WinDBG session with source line numbers is attached

Possibly related to:

https://bugs.webkit.org/show_bug.cgi?id=89658

Long existing bug in Qt Windows

Happens when deleting multiple PNG image files referenced in ebook page in QWebView widget sometime after calling QWebSettings::clearMemoryCaches
Simply trying to update main window status bar with a message that the files were deleted.

I would be happy to run further tests or generate new traces for you if you think it would help. I have not built Qt from source, instead I am using the Qt Installer version for Win32 opengl debug libraries.