Caused by https://bugs.webkit.org/show_bug.cgi?id=80992. This happens because pageDebuggerAgent is not set on instrumenting agents in InspectorDebuggerAgent::restore(). I think the fix would be to make InspectorDebuggerAgent::enable() virtual and overriden in PageDebuggerAgent instead of InspectorDebuggerAgent::enable(ErrorString*)
*** This bug has been marked as a duplicate of bug 106555 ***