Cherry-pick c3b754f. rdar://118486861

    Crash under WebPage::close()
    https://bugs.webkit.org/show_bug.cgi?id=269373
    rdar://118486861

    Reviewed by Brent Fulgham.

    Add a null check for the LocalFrame given that nothing prevents it
    from being null and we're seeing null dereferences in the wild.

    * Source/WebKit/WebProcess/WebPage/WebPage.cpp:
    (WebKit::WebPage::close):

    Canonical link: https://commits.webkit.org/274641@main

Identifier: 272448.609@safari-7618-branch

Source/WebKit/WebProcess/WebPage/WebPage.cpp

@@ -1791,7 +1791,8 @@ void WebPage::close()
 #endif
 
     m_printContext = nullptr;
-    m_mainFrame->coreLocalFrame()->loader().detachFromParent();
+    if (RefPtr localFrame = m_mainFrame->coreLocalFrame())
+        localFrame->loader().detachFromParent();
 
 #if ENABLE(SCROLLING_THREAD)
     if (m_useAsyncScrolling)