Thanks for your hint. Meanwhile I debugged the code and found the reason of 
the NullPointerException.

The method getObjectById is implemented by the class 
DatastoreJDOPersistenceManager. Within this method there is 
*EntityUtils.idToInternalKey(getObjectManager(), 
cls, key, false);*. This methods ends (obviously) in the class EntityUtils. 
Within this class and method you call *determineKind(cmd, om);*. But the 
parameter cmd is alrady null. A few methods later there is the follwoing 
line: *if (acmd.getTable() != null) {...}* et voila -> NullPointerException.

The reason why acmd is null is because of this line within the EntityUtils 
class (method idToInternalKey): 
*AbstractClassMetaData cmd = 
om.getMetaDataManager().getMetaDataForClass(cls, 
om.getClassLoaderResolver());*
Within the method getMetaDataForClass there is the following check:
*if (isClassWithoutPersistenceInfo(c.getName())){ return null;}*

So, does anybody know what this means?
Yes - my entity class does not have any annotation but I thought this isn't 
necessary at all?! Should I have to include such kind of informations to my 
class?

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