A mouse click in the window will trigger the `VIEW3D_OT_cursor3d` operator
before the viewport is available. This causes a segfault in
`GPU_viewport_engines_data_validate()`.
Other callers of `WM_draw_region_get_viewport()` already check for `NULL`
being returned and handle it gracefully.
Reviewed By: jbakker, fclem
Differential Revision: https://developer.blender.org/D5216
12ceea0434