MapGuide: External Provider widget doesn't work after refresh map by Map Widget

[wuma]

Steps:

1. Open one flexible web layout with commercial layers.
2. Click the Map Widget and select the map.
3. Click External Provider widget.

Results: In IE, nothing happened. In FF, commercial layers were listed in the left side but cannot be selected.

Expected results: External widget should work as usual.

[wuma]

r2527