I've seen the QWebEngineProfile() no-arguments constructor crash on several Windows 10 machines using Qt 5.12.10. The call stack is:

  Qt5WebEngineCore.dll!base::Value::GetAsDictionary(base::DictionaryValue * * out_value) Line 520
  Qt5WebEngineCore.dll!catalog::RetrieveRequiredFiles(const base::Value & manifest_value) Line 44
  Qt5WebEngineCore.dll!content::`anonymous namespace'::BuiltinManifestProvider::AddServiceManifest(base::BasicStringPiece<std::basic_string<char,std::char_traits<char>,std::allocator<char> > > name, int resource_id) Line 228
  Qt5WebEngineCore.dll!content::ServiceManagerContext::ServiceManagerContext(scoped_refptr<base::SingleThreadTaskRunner> service_manager_thread_task_runner) Line 470
  Qt5WebEngineCore.dll!content::BrowserMainLoop::InitializeMojo() Line 1597 C++
  Qt5WebEngineCore.dll!content::BrowserMainLoop::BrowserThreadsStarted() Line 1254
  [Inline Frame] Qt5WebEngineCore.dll!base::RepeatingCallback<int __cdecl(void)>::Run() Line 129
  Qt5WebEngineCore.dll!content::StartupTaskRunner::RunAllTasksNow() Line 43
  Qt5WebEngineCore.dll!content::BrowserMainLoop::CreateStartupTasks() Line 933
  Qt5WebEngineCore.dll!content::BrowserMainRunnerImpl::Initialize(const content::MainFunctionParams & parameters) Line 144
  Qt5WebEngineCore.dll!QtWebEngineCore::WebEngineContext::WebEngineContext() Line 574
  Qt5WebEngineCore.dll!QtWebEngineCore::WebEngineContext::current() Line 284
  Qt5WebEngineCore.dll!QtWebEngineCore::ProfileAdapter::ProfileAdapter(const QString & storageName) Line 87
  Qt5WebEngineWidgets.dll!QWebEngineProfile::QWebEngineProfile(QObject * parent) Line 289

I can see that it crashes in qtwebengine\src\3rdparty\chromium\base\values.cc in Value::GetAsDictionary because 'this' is nullptr. That means that the 'mainfest_value' is nullptr in qtwebengine\src\3rdparty\chromium\content\browser\service_manager\service_manager_context.cc:

std::unique_ptr<base::Value> manifest_value =
        base::JSONReader::Read(contents);
    DCHECK(manifest_value);

I assume this means the 'contents' string is empty or invalid.