# Objective
fix an occasional crash when moving ui root nodes between cameras.
occasionally, updating the TargetCamera of a ui element and then
removing the element causes a crash.
i believe that is because when we assign a child in taffy, the old
parent doesn't remove that child from it's children, so we have:
```
user: create root node N1, camera A
-> layout::set_camera_children(A) :
- create implicit node A1
- assign 1 as child -> taffy.children[A1] = [N1], taffy.parents[1] = A1
user: move root node N1 to camera B
-> layout::set_camera_children(B) :
- create implicit node B1
- assign 1 as child -> taffy.children[A1] = [N1], taffy.children[B1] = [N1], taffy.parents[1] = B1
-> layout::set_camera_children(A) :
- remove implicit node A1 (which still has N1 as a child) ->
-> taffy sets parent[N1] = None ***
-> taffy.children[B1] = [N1], taffy.parents[1] = None
user: remove N1
-> layout::remove_entities(N1)
- since parent[N1] is None, it's not removed from B1 -> taffy.children[B1] = [N1], taffy.parents[1] is removed
-> layout::set_camera_children(B)
- remove implicit node B1
- taffy crash accessing taffy.parents[N1]
```
## Solution
we can work around this by making sure to remove the child from the old
parent if one exists (this pr).
i think a better fix may be for taffy to check in `Taffy::remove` and
only set the child's parent to None if it is currently equal to the node
being removed but i'm not sure if there's an explicit assumption we're
violating here (@nicoburns).
30940e5cb4