Expected Behavior

Clicking on a custom procedure block should not cause a type error.

Actual Behavior

Sometimes, clicking on a custom procedure block of an imported SB2 project causes a type error:

raven.js:429 Uncaught TypeError: Cannot read property 'opcode' of undefined
    at e.value (lib.min.js:formatted:9160)
    at e.value (lib.min.js:formatted:22148)
    at e.value (lib.min.js:formatted:22424)
    at e.value (lib.min.js:formatted:28412)

seems to be coming from this line of code.

Steps to Reproduce

1. Open up project #10128407 in scratch 3.0.
2. Switch to the "Steve" sprite
3. In the blocks toolbox, click on the "PositionShow_S" block.

Needs a more minimal repro.

Operating System & Browser Info

macOs 10.13.2 and Chrome Version 64.0.3282.119 (Official Build) (64-bit)