tree_view crash

Might be related to #50 

Steps:
1. run debug version of `app`
1. uncollapse `root2`
1. uncollapse `root1`
1. uncollapse `subroot3`
1. uncollapse `subsubroot1`
1. uncollapse `subroot1`
1. delete `subroot3`

Actual result: crash occurs
Expected result: no crash