`outliner_draw_overrides_buts` uses `uiDefIconBlockBut` but doing so
without defining a function callback to actually build a block.
This will make the button go down the route of spawning a popup, but
without a menu. Crash then happens later accesing the (missing) menu in
`ui_handler_region_menu`.
So while we could dive into making this usage failsafe (carefully
checking `BUTTON_STATE_MENU_OPEN` in combination with
`uiHandleButtonData->menu` being NULL all over), but it seems much more
straightforward to just use `uiDefIconBut` (instead of
`uiDefIconBlockBut`) since this Override Warning buttons seem not to
intend spawning a menu anyways?
Maniphest Tasks: T92265
Differential Revision: https://developer.blender.org/D12917
57f1379104