Graph: Add dependecies in tag with a parent freezes graph

[dubmassacre]

Tested in v0.5.5

Steps to reproduce:

1. Create a new folder
2. Start server.
3. Change DB directory to the new folder
4. Launch application
5. Create a tag e.g institution
6. Create a second tag political party with parent tag the institution tag.
7. Click 'Add all dependencies to the graph' for the political party tag
8. In graph, right click political party tag and select 'add dependencies'
9. Graph freezes, showing the following.