“Add circuit blocks” crashes on color sets

[kpreid]

The “Add circuit blocks” function assumes that block type #1 is a world block with an appropriate blockset. This is a bad assumption.

We could search for the first available world block. However, that would fail given e.g. a pure color blockset. Perhaps a more robust solution, and one more amenable to user understanding of the system state, would be to require all world blocks in a given blockset to be composed of the same blockset (as each other), which is specified in the containing blockset.