Fix crash on wxDataViewCtrl creation after r73565.

Initialize m_clientArea to NULL to avoid crash in SetFont() which is called
during the control creation.

This might still be not 100% correct as the initial font is not set at all for
the client area but at least it fixes the crash.

See #15056.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73601 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775