Expected Behavior

• Creating a custom procedure with "%s" as an argument name should work

Actual Behavior

• Causes a number of exceptions that lead to instability

Steps to Reproduce

• Click the "Make a block" button
• Enter %s as the argument name
• Click "ok"

Demo

Operating System and Browser

Mac OS 10.13.2 | Chrome Version 63.0.3239.132 (Official Build) (64-bit)

Moved from scratchfoundation/scratch-gui#1347
/cc @thisandagain @Kenny2github @amazinigmech2418