Gallery2: force close happens when remove Gallery widget

The mSource may be null if we destroyed the widget.

So we make the function return null if we remove the
widget to make gallery avoid force closing.

CRs-Fixed: 362477

Change-Id: Ied227235f4dc78aeed69a70c264e523268c344d0
Merge "Gallery2: force close happens when remove Gallery widget" into du44