Reset sorting column index in generic wxDataViewCtrl when clearing columns.

Otherwise we could continue to use the now invalid index for sorting.

Closes #15803.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75533 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/datavgen.cpp

@@ -5016,6 +5016,7 @@ bool wxDataViewCtrl::ClearColumns()
 {
     SetExpanderColumn(NULL);
     m_cols.Clear();
+    m_sortingColumnIdx = wxNOT_FOUND;
     m_colsBestWidths.clear();
 
     m_clientArea->ClearCurrentColumn();