This was working differently in 2.79, tried tracking this down and it
seems this was wrong since the 2.8 beginning in rB7907dfc40018: Motion Tracking Reconstruction Viewport Drawing (T51544).
This would not only crash without an active scene camera, but would also
result in different tracks from different camera's constraints not being selectable.
So select id depends on corresponding camera, remove the dependency on
scene camera completely.