Error after the copy of XWiki.ClassSheet to a different space

Steps to reproduce:

• Go on the XWiki.ClassSheet page of your wiki.
• Select the copy option from the menu Page: ClassSheet > Copy
• Fill in the target space and click the "Copy" button
  Result: the page is not copied and a velocity errors are displayed

  Wrapped Exception:
  
  org.apache.velocity.exception.MethodInvocationException: Invocation of method 'copyDocument' in  class com.xpn.xwiki.api.XWiki threw exception com.xpn.xwiki.XWikiException: Error number 3301 in 3: Exception while switching to database xwiki
  Wrapped Exception: null at /templates/copy.vm[line 51, column 18]
  	at org.apache.velocity.runtime.parser.node.ASTMethod.handleInvocationException(ASTMethod.java:243)
  	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:187)
  ...
  Caused by: com.xpn.xwiki.XWikiException: Error number 3301 in 3: Exception while switching to database xwiki
  Wrapped Exception: null
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:661)
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:832)
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.beginTransaction(XWikiHibernateBaseStore.java:732)
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.execute(XWikiHibernateBaseStore.java:1220)
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.executeWrite(XWikiHibernateBaseStore.java:1307)
  	at com.xpn.xwiki.store.XWikiHibernateVersioningStore.saveXWikiDocArchive(XWikiHibernateVersioningStore.java:175)
  	at com.xpn.xwiki.XWiki.copyDocument(XWiki.java:4062)
  	at com.xpn.xwiki.api.XWiki.copyDocument(XWiki.java:1491)
  	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
  	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
  	at java.lang.reflect.Method.invoke(Method.java:597)
  	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.doInvoke(UberspectImpl.java:395)
  	at org.apache.velocity.util.introspection.UberspectImpl$VelMethodImpl.invoke(UberspectImpl.java:384)
  	at org.apache.velocity.runtime.parser.node.ASTMethod.execute(ASTMethod.java:173)
  	... 57 more
  Caused by: java.lang.NullPointerException
  	at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getXWikiContext(AbstractDataMigrationManager.java:284)
  	at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.getDataMigrationStatus(AbstractDataMigrationManager.java:406)
  	at com.xpn.xwiki.store.migration.AbstractDataMigrationManager.checkDatabase(AbstractDataMigrationManager.java:520)
  	at com.xpn.xwiki.store.XWikiHibernateBaseStore.setDatabase(XWikiHibernateBaseStore.java:657)