Missing steps cause steps to be shifted and the GUI to crash
If some larvae are tracked with abnormal time gaps, while other larvae are defined at the missing time steps, the GUI crashes on saving the affected tracks to file, or on selecting a time step that is normally defined but comes shifted down in time because steps are encoded as indices with contiguity assumed.
The contiguity assumption is valid with data derived from MWT, and a crash is almost desirable here. However, the above issue recently arose with data acquired using a new unpublished tracker. Therefore the assumption has to be dropped now.