DevTools: Fix exception on timeline clear

third_party/WebKit/Source/devtools/front_end/timeline/TimelinePanel.js

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  * Copyright (C) 2012 Intel Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 608 matching lines @@
 
   _toggleRecording() {
     if (this._state === Timeline.TimelinePanel.State.Idle)
       this._startRecording(true);
     else if (this._state === Timeline.TimelinePanel.State.Recording)
       this._stopRecording();
   }
 
   _clear() {
     this._showLandingPage();
-    this._detailsSplitWidget.hideSidebar();
+    if (this._detailsSplitWidget)
+      this._detailsSplitWidget.hideSidebar();
     this._sessionGeneration = null;
     this._recordingStartTime = 0;
     this._reset();
   }
 
   _reset() {
     PerfUI.LineLevelProfile.instance().reset();
     this._tracingModel.reset();
     this._model.reset();
     for (let extensionEntry of this._extensionTracingModels)
@@ skipping 909 matching lines @@
         return true;
       case 'timeline.jump-to-next-frame':
         panel._jumpToFrame(1);
         return true;
     }
     return false;
   }
 };
 
 Timeline.TimelinePanel._traceProviderSettingSymbol = Symbol('traceProviderSetting');