an unxpected error occured- while trying to make blocks

When going ' block' selecting the symbol I want to make a block, then close and save, 'an expected error occured. the application became unstable' comes up and it doesn't save the block.

But most of the time this doesn't happen and it makes the block perfectly, Very strange.

But once it has happened once to that symbol, it happens every time I try to make it into a block.