fix crash which happened if you called SetAttr(NULL) followed by SetAttr(attr) (#9567)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@54176 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/grid.cpp

@@ -2590,8 +2590,12 @@ void wxGridCellAttrData::SetAttr(wxGridCellAttr *attr, int row, int col)
     int n = FindIndex(row, col);
     if ( n == wxNOT_FOUND )
     {
-        // add the attribute
+        if ( attr )
-        m_attrs.Add(new wxGridCellWithAttr(row, col, attr));
+        {
+            // add the attribute
+            m_attrs.Add(new wxGridCellWithAttr(row, col, attr));
+        }
+        //else: nothing to do
     }
     else
     {