Sanity check for Transform.select

[mlajtos]

Problem
Transform.select(editor: Editor, target: Location) will use whatever target it is provided without checking it actually exists or if it points to leaf nodes.

Solution
Adding code that checks whether the target location exists and it is indeed leaf node. If it is not leaf node, it will use Editor.first(editor: Editor, at: Location).

Alternatives
Not adding code for checking. User might be surprised when s/he tries to select node at [0] and editor will crash or do something unexpected.