53061: "Clear mesh data" removes too many (if !HasShapeToMesh())

doc/salome/gui/SMESH/input/about_meshes.doc

@@ -110,9 +110,10 @@ SALOME supports elements of second order, without a central node
 (quadratic triangle, quadrangle, polygon, tetrahedron, hexahedron,
 pentahedron and pyramid) and with central nodes (bi-quadratic triangle
 and quadrangle and tri-quadratic hexahedron).<br>
-Quadratic mesh can be obtained in two ways:
+Quadratic mesh can be obtained in three ways:
 - Using a global \ref quadratic_mesh_anchor "Quadratic Mesh"
 hypothesis. (Elements with the central node are not generated in this way).
 - Using \ref convert_to_from_quadratic_mesh_page operation.
-
+- Using an appropriate option of some meshing algorithms, which
+generate elements of several dimensions starting from mesh segments.
 */

src/SMESH_I/SMESH_Mesh_i.cxx

@@ -293,7 +293,7 @@ void SMESH_Mesh_i::Clear() throw (SALOME::SALOME_Exception)
 {
   Unexpect aCatch(SALOME_SalomeException);
   if ( _preMeshInfo )
-    _preMeshInfo->ForgetAllData();
+    _preMeshInfo->ForgetOrLoad(); // load in case if !HasShapeToMesh()
 
   try {
     _impl->Clear();