SALOME Forum bug: sub-mesh removal leads to an Exception in a re-opened study.

SALOME Forum bug: sub-mesh removal leads to an Exception in a re-opened study.

Now UnRegister(), corresponding to the Register() called by AttributeIOR,
is invoked by an object publisher i.e. by SMESH_Gen_i::publish() but not by GUI
because GUI is unable to track publication of all objects created by the Engine:
for example groups can be created while mesh.Compute() is called.