(3rd part)
"Bug Report:"
thanks for reporting this.
It turns out in this particular case, what happens is when you paste nametable to chr (nexxt accepting this is relatively new - it works by looking up what tiles the nametable references, and then copy those tiles to the new position in the tileset), having a copy selection larger than can fit 256 tiles will crash, because the copy buffer never was designed to handle copies larger than one tile set. I've increased the buffer to deal with 4x more tiles gracefully, and if you exceed *that* it will simply warn you and not proceed. But this should be difficult to even encounter.
So, this bug should be resolved now.