Bug 248752

As per https://ews-build.s3-us-west-2.amazonaws.com/macOS-AppleSilicon-Big-Sur-Debug-WK2-Tests-EWS/3b932e85-45320/media/video-inactive-playback-crash-log.txt, we can see the following issue:
ASSERTION FAILED: m_element
/Volumes/Data/worker/macOS-AppleSilicon-Big-Sur-Debug-Build-EWS/build/Source/WebKit/WebProcess/FullScreen/WebFullScreenManager.cpp(293) : void WebKit::WebFullScreenManager::willExitFullScreen()
1   0x144c3c960 WTFCrash
2   0x1053ee6f0 WTFCrashWithInfo(int, char const*, char const*, int)
3   0x106fedc4c WebKit::WebFullScreenManager::willExitFullScreen()
4   0x10733d578 WKBundlePageWillExitFullScreen
5   0x167052490 WTR::InjectedBundlePage::exitFullScreenForElement(OpaqueWKBundleNodeHandle const*)
6   0x16704b3f8 WTR::InjectedBundlePage::exitFullScreenForElement(OpaqueWKBundlePage const*, OpaqueWKBundleNodeHandle const*)
7   0x10732bbd4 WebKit::InjectedBundlePageFullScreenClient::exitFullScreenForElement(WebKit::WebPage*, WebCore::Element*)
8   0x106fed180 WebKit::WebFullScreenManager::exitFullScreenForElement(WebCore::Element*)
9   0x1075379ac WebKit::WebFrameLoaderClient::dispatchDidStartProvisionalLoad()
10  0x11f718c98 WebCore::FrameLoader::prepareForLoadStart()
11  0x11f761368 WebCore::FrameLoader::continueLoadAfterNavigationPolicy(WebCore::ResourceRequest const&, WebCore::FormState*, WebCore::NavigationPolicyDecision, WebCore::AllowNavigationToInvalidURL)::$_11::operator()()
12  0x11f76114c WTF::Detail::CallableWrapper<WebCore::FrameLoader::continueLoadAfterNavigationPolicy(WebCore::ResourceRequest const&, WebCore::FormState*, WebCore::NavigationPolicyDecision, WebCore::AllowNavigationToInvalidURL)::$_11, void>::call()
13  0x11dade974 WTF::Function<void ()>::operator()() const
14  0x11ae4d194 WTF::CompletionHandler<void ()>::operator()()
15  0x11f71dd90 WebCore::FrameLoader::continueLoadAfterNavigationPolicy(WebCore::ResourceRequest const&, WebCore::FormState*, WebCore::NavigationPolicyDecision, WebCore::AllowNavigationToInvalidURL)
16  0x11f75d568 WebCore::FrameLoader::loadWithDocumentLoader(WebCore::DocumentLoader*, WebCore::FrameLoadType, WTF::RefPtr<WebCore::FormState, WTF::RawPtrTraits<WebCore::FormState>, WTF::DefaultRefDerefTraits<WebCore::FormState> >&&, WebCore::AllowNavigationToInvalidURL, WTF::CompletionHandler<void ()>&&)::$_8::operator()(WebCore::ResourceRequest const&, WTF::WeakPtr<WebCore::FormState, WTF::DefaultWeakPtrImpl>&&, WebCore::NavigationPolicyDecision)
17  0x11f75d408 WTF::Detail::CallableWrapper<WebCore::FrameLoader::loadWithDocumentLoader(WebCore::DocumentLoader*, WebCore::FrameLoadType, WTF::RefPtr<WebCore::FormState, WTF::RawPtrTraits<WebCore::FormState>, WTF::DefaultRefDerefTraits<WebCore::FormState> >&&, WebCore::AllowNavigationToInvalidURL, WTF::CompletionHandler<void ()>&&)::$_8, void, WebCore::ResourceRequest&&, WTF::WeakPtr<WebCore::FormState, WTF::DefaultWeakPtrImpl>&&, WebCore::NavigationPolicyDecision>::call(WebCore::ResourceRequest&&, WTF::WeakPtr<WebCore::FormState, WTF::DefaultWeakPtrImpl>&&, WebCore::NavigationPolicyDecision)