If no history is returned, empty out the history event list

* Otherwise it stays as "Loading..." forever

renderdocui/Windows/PixelHistoryView.cs

@@ -398,12 +398,16 @@ namespace renderdocui.Windows
 
         private void UpdateEventList()
         {
-            if (modifications == null) return;
-
             events.BeginUpdate();
 
             events.Nodes.Clear();
 
+            if (modifications == null)
+            {
+                events.EndUpdate();
+                return;
+            }
+
             var frags = new List<TreelistView.Node>();
             var mods = new List<PixelModification>();