Crash when pasting custom instruments in a song with no custom instruments

[Bentroen]

If you copy a note block which is a custom instrument, then make a new song, paste it, and then play the song, the program crashes.

Likely, the instrument ID becomes out-of-range, and somewhere down where NBS processes the pasted note blocks, it tries to read an invalid array index.

This behavior will likely not be patched in the current version, but we should make sure a failsafe is in place for the rewrite (either prevent the selection from being pasted with a popup warning, or create empty custom instruments up until the largest index present on the selection).

[Bentroen]

Window with the traceback posted by @BedrRedstone, which should make it easier to track where the issue happens:

Fixing this should be a matter of surrounding this operation in a try/catch block and ignoring the notes with missing instruments, with a notice at the end.

[Bentroen]

It also should be noted that the issue happens when clearing the affected selection. It's possible to paste the selection normally, but the notes with missing instruments just appear to be invisible.