DialogFragement: Add null pointer check for mDialog for protection

DialogFragement can be dissmissed just after the Dialog is
established. This will cause NullPointerException.

Add null pointer check for mDialog for protection

CRs-fixed: 549689

Change-Id: If57f81d03a8ec2906598363b681ea38806fb2a90
Merge "DialogFragement: Add null pointer check for mDialog for protection" into du44