Segmentation fault when trying to indent

[Compro-Prasad]

Steps for reproduction:

1. Run thittam.
2. Click on + to create two tasks in the top level.
3. Click on the second task or node.
4. Click on the indent button on the toolbar.
5. Click on the unindent button on the toolbar.
6. Click on the indent button again.
7. Go to step 4 and repeat the steps until a segmentation fault occurs.

[Compro-Prasad]

https://github.com/froyg/thittam/pull/26