Multiple items edition is malfunctioning

[hsilva-keep]

RODA-in version: 2.0.0-beta5

How to reproduce:

1. Create a root node
2. Create a child note beneath the previously created node
3. Select both nodes to activate multiple edition
4. Don't edit anything and deselect the nodes selection by clicking in the root node

Problem:
Now the root node ID is {{mixed}} (and previously was a random UUID). Also, the node lost the level attribute as well (if one changes the view to text, the level also is set to {{mixed}})