Fix crash if a marker group has children but doesn't have any nodes

* This could happen if the option to remove empty groups was enabled,
  and all the children in a group were removed.

renderdocui/Windows/EventBrowser.cs

@@ -301,7 +301,7 @@ namespace renderdocui.Windows
                     }
                 }
 
-                if(!found)
+                if (!found && !drawNode.Nodes.IsEmpty())
                     drawNode.Tag = drawNode.Nodes.LastNode.Tag;
             }