Fix a random crash in the TileSet editor
Fix a random crash in the TileSet editor

Occurred when selecting polygon collision mode, then drawing a single point, then selecting rectangle mode, and finally trying to select a rectangle.

(cherry picked from commit 7850d0316f1100eb0f6b4229fb76784ea20a3e75)

ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot,ex/godot