missing grid lines if two image cubes with different channel numbers are opened

[kswang1029]

To reproduce the issue, firstly open orion_12co_hera.fits (35 channels) and then open M100_alma.fits (70 channels). Now activate M100_alma.fits with the animator panel. Play the movie and keep an eye on the channel numbers between 34 and 35. Starting from channel "35", the grid lines disappear.

If we open M100_alma.fits first and then orion_12co_hera.fits, this issue will not be visible since the first cube has more channels than the second one.

[slovelan]

This should be fixed with my next check-in.