Fix crash in ExtensionPopup::DevToolsAgentHostDetached upon extension unload

ExtensionPopup::OnExtensionUnloaded() deletes the ExtensionHost,
we destroy the WebContents, this leads to detaching of DevTools
sessions which will be reported to
ExtensionPopup::DevToolsAgentHostDetached where we access host.
Oops.

Bug: 1052057
Change-Id: I34e03582beb61c0308ad35bc973dc5cd4e26a5b8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2066506
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Johannes Henkel <johannes@chromium.org>
Reviewed-by: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746168}

chrome/browser/ui/views/extensions/extension_popup.cc

@@ -167,6 +167,12 @@ void ExtensionPopup::DevToolsAgentHostAttached(
 
 void ExtensionPopup::DevToolsAgentHostDetached(
     content::DevToolsAgentHost* agent_host) {
+  // If the extension's page is open it will be closed when the extension
+  // is uninstalled, and if DevTools are attached, we will be notified here.
+  // But because OnExtensionUnloaded was already called, |host_| is
+  // no longer valid.
+  if (!host())
+    return;
   if (host()->host_contents() == agent_host->GetWebContents())
     show_action_ = SHOW;
 }