QWindowsScreenManager::handleScreenChanges crashes when WindowsTheme is not used

When not using WindowsTheme, but a custom theme, QWindowsScreenManager::handleScreenChanges() crashes when WM_SETTINGCHANGE is received (e.g. when the taskbar is set to auto-hide off/on).

This is because QWindowsTheme::instance()->refreshFonts() is called without checking whether the instance is valid.

I believe it should be checked for NULL like it is on two places in QWindowsContext::windowsProc.