How this seems to happen for me: 1. Run debug build 2. Open Inspector 3. Inspector process crashes (for me, at ASSERTION FAILED: m_cachedThreadGlobalTimerHeap) (?) 4. UIProcess tries to close the WKWebView after a timeout (?) 5. Crash Relevant call frames: Thread 0 Crashed:: Dispatch queue: com.apple.main-thread 0 com.apple.JavaScriptCore 0x000000010099323a WTFCrash + 42 1 com.apple.WebKit 0x000000010224d7cb WebKit::PageLoadState::removeObserver(WebKit::PageLoadState::Observer&) + 91 (PageLoadState.cpp:78) 2 com.apple.WebKit 0x0000000102162c57 WebKit::NavigationState::~NavigationState() + 391 (NavigationState.mm:97) 3 com.apple.WebKit 0x0000000102162cb5 WebKit::NavigationState::~NavigationState() + 21 (NavigationState.mm:98) 4 com.apple.WebKit 0x0000000102162cd9 WebKit::NavigationState::~NavigationState() + 25 (NavigationState.mm:93) 5 com.apple.WebKit 0x00000001029bc895 -[WKWebView .cxx_destruct] + 565 (memory:2625) 6 libobjc.A.dylib 0x00007fff8d5a1ae8 object_cxxDestructFromClass(objc_object*, objc_class*) + 127 7 libobjc.A.dylib 0x00007fff8d59c637 objc_destructInstance + 119 8 libobjc.A.dylib 0x00007fff8d59c595 object_dispose + 22 9 com.apple.AppKit 0x00007fff97a1bb8c -[NSResponder dealloc] + 139 10 com.apple.AppKit 0x00007fff97a19bfd -[NSView dealloc] + 182 11 com.apple.WebKit 0x00000001029ba7e5 -[WKWebView dealloc] + 309 (WKWebView.mm:392) 12 libobjc.A.dylib 0x00007fff8d5b689c objc_object::sidetable_release(bool) + 236 13 com.apple.CoreFoundation 0x00007fff8e3f09c0 CFRelease + 304 14 com.apple.WebKit 0x000000010262b6fe WTF::RetainPtr<WKWebInspectorWKWebView>::~RetainPtr() + 46 (RetainPtr.h:74) 15 com.apple.WebKit 0x0000000102625ae5 WTF::RetainPtr<WKWebInspectorWKWebView>::~RetainPtr() + 21 (RetainPtr.h:74) 16 com.apple.WebKit 0x00000001026325ea WTF::RetainPtr<WKWebInspectorWKWebView>::operator=(WKWebInspectorWKWebView*) + 74 (RetainPtr.h:187) 17 com.apple.WebKit 0x000000010262e10a WebKit::WebInspectorProxy::closeTimerFired() + 298 (WebInspectorProxyMac.mm:318) 18 com.apple.WebKit 0x000000010262b97e WTF::RunLoop::Timer<WebKit::WebInspectorProxy>::fired() + 110 (RunLoop.h:121) 19 com.apple.JavaScriptCore 0x00000001009cf2a9 WTF::RunLoop::TimerBase::timerFired(__CFRunLoopTimer*, void*) + 57 20 com.apple.CoreFoundation 0x00007fff8e48db64 __CFRUNLOOP_IS_CALLING_OUT_TO_A_TIMER_CALLBACK_FUNCTION__ + 20 21 com.apple.CoreFoundation 0x00007fff8e48d7f3 __CFRunLoopDoTimer + 1059 22 com.apple.CoreFoundation 0x00007fff8e500dbd __CFRunLoopDoTimers + 301 23 com.apple.CoreFoundation 0x00007fff8e44a288 __CFRunLoopRun + 2024 24 com.apple.CoreFoundation 0x00007fff8e449858 CFRunLoopRunSpecific + 296 25 com.apple.HIToolbox 0x00007fff8c070aef RunCurrentEventLoopInMode + 235 26 com.apple.HIToolbox 0x00007fff8c07086a ReceiveNextEventCommon + 431 27 com.apple.HIToolbox 0x00007fff8c0706ab _BlockUntilNextEventMatchingListInModeWithFilter + 71 28 com.apple.AppKit 0x00007fff979d4f81 _DPSNextEvent + 964 29 com.apple.AppKit 0x00007fff979d4730 -[NSApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 194 30 com.apple.Safari.framework 0x00007fff8a884710 -[BrowserApplication nextEventMatchingMask:untilDate:inMode:dequeue:] + 246 31 com.apple.AppKit 0x00007fff979c8593 -[NSApplication run] + 594 32 com.apple.AppKit 0x00007fff979b3a14 NSApplicationMain + 1832 33 libdyld.dylib 0x00007fff8d5915c9 start + 1
<rdar://problem/19861472>