Fix pattern length crash bug

[Andre Wichmann]

"I had some weird crashes still at the same moment : first the R2 pattern was 32 long, then i decided to make it 64, so I used insert to push lines, and after, each time i wanted to put a new note, it crashed... It worked when I inserted more L2 patterns first and then push lines in R2 to be longer."

[Andre Wichmann]

crash fixed