view all structures does not always superpose structures

selecting 'View all structures' on an alignment with 2 or more structures available doesn't always result in a view where the structures are always superposed.

If the sequence under the mouse pointer when the menu item was selected has a PDB ID, it is used as the reference structure, and superposition works. If there is no associated structure to be used as reference, the superposition does not take place and exceptions are raised.