Song length stays the same even though I remove patterns from the arrangement

[vlandero]

To reproduce:

You have an arrangement, let's say of 2 minutes. Then, you delete all the patterns from the "patterns palette" up right, they are removed from the arrangement (or maybe I think there are null pointers here :D). Then, you create a new pattern, you add it to the arrangement, and now you have a song with only one pattern that lasts a couple of seconds, but the song is still 2 minutes. If you try to export it, it will be a 2 minutes empty song. It's the same if you double click on the only pattern in the arrangement (to play the whole song), it will play for 2 minutes with no other patterns.