NPE when importing steps from the same manual test

Steps to reproduce:

• Create a test case with some test steps
• Click on import test steps (from test)
• Choose via dropdown the same test
• Enable checkbox for some steps
• Additionally, click on Reset Test steps
• Click on Add

You'll get the below error and all the Manual Steps in the Test will be erased:

java.lang.NullPointerException
at com.xpandit.raven.service.impl.TestStepServiceImpl.clone(TestStepServiceImpl.java:153)

(...)