Did not encounter this in 5.0.5.x, but try the following:
- Delete the native folder
- Close Eclipse
- Locate your temp folder (Windows, OS X)
- Delete the
wlBuildResources
and wlPreview folders - Open Eclipse
- Re-build
If you have custom native code in your native\ folder, be sure to back it up.
More here (although it was written about Worklight 6.x, should work in 5.x...): Worklight 6.1 - "All Build" fails and gives java.lang.NullPointerException