Don't leave dandling pointers in wxDataViewCtrl in wxOSX.

The previous m_ModelNotifier became invalid but wasn't always replaced with a
valid one, resulting in a crash due to the use of an invalid pointer.

Closes #15910.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75707 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775