EXTJS 4.0 grid in an inactive tab shows nothing

I create a tab panel with two tabs, first one is active and second isn't.

Then I create two grids and render them to first tab content and second tab content.

The first tab shows fine, and the inactive one shows nothing but 开发者_Python百科only the gird layout appears.

I tried to put two grids into the active tab, and it works.

But I want each tab to contain one grid, how can I achieve this?