Fix GetTileGroup bug. Should return nullptr when group not exist.
Fix GetTileGroup bug. Should return nullptr when group not exist.


Former-commit-id: eacb51f5f1ea489992b7e6d286f3eb41649317ef
larryxiao/peloton,ranxian/peloton,ranxian/peloton,amaliujia/CMUDB-peloton,ranxian/peloton,amaliujia/CMUDB-peloton,ranxian/peloton,omegaga/peloton,amaliujia/CMUDB-peloton,omegaga/peloton,larryxiao/peloton,omegaga/peloton,omegaga/peloton,larryxiao/peloton,omegaga/peloton,amaliujia/CMUDB-peloton,ranxian/peloton,larryxiao/peloton-1,larryxiao/peloton-1,larryxiao/peloton-1,larryxiao/peloton,larryxiao/peloton,amaliujia/CMUDB-peloton,ranxian/peloton,larryxiao/peloton-1,omegaga/peloton,amaliujia/CMUDB-peloton,larryxiao/peloton-1,larryxiao/peloton,omegaga/peloton,amaliujia/CMUDB-peloton,larryxiao/peloton-1,larryxiao/peloton-1,larryxiao/peloton