DevTools: fix Audits panel run

BUG=741310

Change-Id: I324c0403af2d351373536897a7f2bc282c870e41
Reviewed-on: https://chromium-review.googlesource.com/568682Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Patrick Hulce <phulce@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486136}

third_party/WebKit/Source/devtools/front_end/audits2/Audits2Panel.js

@@ -112,6 +112,9 @@ Audits2.Audits2Panel = class extends UI.PanelWithSidebar {
    * @return {?string}
    */
   _unauditablePageMessage() {
+    if (!this._manager)
+      return null;
+
     var inspectedURL = SDK.targetManager.mainTarget().inspectedURL();
     if (/^about:/.test(inspectedURL))
       return Common.UIString('Cannot audit about:* pages. Navigate to a different page to start an audit.');