Reset the expander column when clearing columns in generic wxDVC.

The expander column becomes invalid when all columns are cleared, don't keep
a dangling pointer.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69165 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/datavgen.cpp

@@ -4357,6 +4357,7 @@ bool wxDataViewCtrl::DeleteColumn( wxDataViewColumn *column )
 
 bool wxDataViewCtrl::ClearColumns()
 {
+    SetExpanderColumn(NULL);
     m_cols.Clear();
     m_colsBestWidths.clear();
     OnColumnsCountChanged();