In some cases, when Datanucleus Enhancer throws exception as follows:

An internal error occurred during: “DataNucleus Enhancer”. java.lang.NullPointerException

If you see such error that means that the Enhancer cannot traverse through the source tree to find the classes because the tree was too large. In such case, you can add the path to the data classes in the Google App Engine configuration window as follows:

1)Bring up your Google App Engine setting window

2)click on “ORM” choice: – click on the src and edit the path to the data classes package.

3) Save it and do your Datanucleus Enhancer.

Here is the URL for bug report
http://code.google.com/p/googleappengine/issues/detail?id=1862