MdeModulePkg/UiApp: Fix the incorrect use of the HiiHandle
In current code, when adding string package, it will return
'gStringPackHandle'. But the code use the 'gHiiHandle' to get
string. It is incorrect. This patch is to fix this issue.
Cc: Eric Dong <eric.dong@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Dandan Bi <dandan.bi@intel.com> Reviewed-by: Eric Dong <eric.dong@intel.com>