This fixes the crash by removing the `do_view3d_header_buttons` handler.
The code can work at a higher level here, using the operator for setting
the select mode, which makes this patch a cleanup as well.
The operator now has a description callback to add the custom
description used for the behavior in its invoke method.
Differential Revision: https://developer.blender.org/D13660
b73d3b80fd