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User-Agent: Mozilla/5.0 Galeon/1.2.9 (X11; Linux i686; U;) Gecko/20030314

Description of problem:
There are two bits of code in DomainObjectFactory which do not handle nulls
correctly. This has two effects:

 * It is possible to register an instantiator against a 'null' object type.
 * When recursively looking up instantiators for parent objects types, it will
eventually look up an instantiator for a 'null' object type.

If something accidently registers an instantiator against 'null' type (eg
through typo in the object type string they pass in), then anything which
doesn't have an explicit instantantiator registered suddenly resolves to this
instantiator. 

This is already fixed in Troika & Rickshaw, but needs to be backported to 5.2 as
well

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Register an instantiator against a type which doesn't exist
2. Try & instantiate an object for which no instanatiator is found
3.
    

Actual Results:  It tries to use the instantiator registered against 'null'
object type

Expected Results:  It throws an instantiator not found exception.

Additional info:

p4 change 33290 is suitable for direct application to the 5.2 branch