ChildNodeProvider can throw NPE when adding new node, because node of prototype can be null

The child node provider can cause a null pointer exception when adding a new node. When the prototype is not null a copy of the prototype node will be made. But it's not guaranteed that the getNode on the prototype will return a node.

[INFO] [talledLocalContainer] Caused by: java.lang.NullPointerException
[INFO] [talledLocalContainer] at org.hippoecm.repository.impl.SessionDecorator.copy(SessionDecorator.java:334)
[INFO] [talledLocalContainer] at org.hippoecm.repository.decorating.checked.SessionDecorator.copy(SessionDecorator.java:401)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.model.ChildNodeProvider.addNew(ChildNodeProvider.java:84)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.editor.plugins.field.AbstractFieldPlugin.getProvider(AbstractFieldPlugin.java:330)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.editor.plugins.field.AbstractFieldPlugin.onBeforeRender(AbstractFieldPlugin.java:303)
[INFO] [talledLocalContainer] at org.hippoecm.frontend.editor.plugins.field.NodeFieldPlugin.onBeforeRender(NodeFieldPlugin.java:93)
[INFO] [talledLocalContainer] at org.apache.wicket.Component.internalBeforeRender(Component.java:1069)
[INFO] [talledLocalContainer] at org.apache.wicket.Component.beforeRender(Component.java:1103)
[INFO] [talledLocalContainer] at org.apache.wicket.MarkupContainer.onBeforeRenderChildren(MarkupContainer.java:1777)