Timeline: fix exception when updating tree view details and Events tree experiment is off

BUG=

Review URL: https://codereview.chromium.org/1467703003

Cr-Commit-Position: refs/heads/master@{#361156}

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

@@ -185,6 +185,9 @@ WebInspector.TimelineTreeView.prototype = {
 
     _updateDetailsForSelection: function()
     {
+        // FIXME: remove this as we implement details for all modes.
+        if (!this._detailsView)
+            return;
         var selectedNode = this._dataGrid.selectedNode ? /** @type {!WebInspector.TimelineTreeView.GridNode} */ (this._dataGrid.selectedNode)._profileNode : null;
         if (selectedNode === this._lastSelectedNode)
             return;