title: ExtJs: Programly select rows of a GridPanel in an inactive tab not working?
body: <p>Basicly, I have a window contains two tabs, the second tab is a GridPanel and is initially inactive.</p>
  
  <p>I want to programly select two rows of this GridPanel(via CheckboxSelectionModel) by clicking a "show window" button , but found it not working. There will be a javascript error thrown and no rows selected. After I manually clicked the second tab to make the grid visible, everything works well.</p>
  
  <p>what's the most possible mistake I made? 
  I'll paste the code soon if it's not a common mistake for extjs beginner.</p>