FEM: meshtools and dependent modules, add error message for retrieving solid elements if out of range

src/Mod/Fem/femguiobjects/_ViewProviderFemMaterial.py

@@ -204,6 +204,9 @@ class _TaskPanelFemMaterial:
         ref_shty = ''
         for ref in self.references:
             r = FemMeshTools.get_element(ref[0], ref[1])  # the method getElement(element) does not return Solid elements
+            if not r:
+                FreeCAD.Console.PrintError('Problem in retrieving element: {} \n'.format(ref[1]))
+                continue
             # print('  ReferenceShape : ', r.ShapeType, ', ', ref[0].Name, ', ', ref[0].Label, ' --> ', ref[1])
             if not ref_shty:
                 ref_shty = r.ShapeType

src/Mod/Fem/femmesh/meshtools.py

@@ -1449,7 +1449,12 @@ def is_same_geometry(shape1, shape2):
 
 def get_element(part, element):
     if element.startswith('Solid'):
-        return part.Shape.Solids[int(element.lstrip('Solid')) - 1]  # Solid
+        index = int(element.lstrip('Solid')) - 1
+        if index >= len(part.Shape.Solids):
+            FreeCAD.Console.PrintError('Index out of range. This Solid does not exist in the Shape!\n')
+            return None
+        else:
+            return part.Shape.Solids[index]  # Solid
     else:
         return part.Shape.getElement(element)  # Face, Edge, Vertex