MISSING LABEL message shown when you add a new child to a vocabulary

Steps to reproduce:

• Create a Hierarchical Vocabulary
• Add a Root
• Add a Child
• The message MISSING LABEL is shown
• Set the Id and Label of the node
• The message MISSING LABEL is still there after filling both fields.
• Add a Child
• Set the Id and Label of the node
• The message MISSING LABEL is still there after filling both fields.
• ...