WARP in visible stepping

Make a block with a WARP around its definition:
untitled script pic - 2022-01-01T160421.825
turn visible stepping on, drag the slider to the left to make it single stepping, and click on this:
untitled script pic - 2022-01-01T160445.749
Next, change the definition of THING to this while the block is running:
untitled script pic - 2022-01-01T160455.245
The WARP you removed looks like this:
untitled script pic - 2022-01-01T160459.388
Next, press the step button, and Snap! crashes. It doesn't look like it does on my computer at least, but you can't do anything.

Here's the error message in the console:

Uncaught TypeError: Cannot read property 'world' of undefined
    at Process.errorBubble (threads.js:1216)
    at Process.throwError (threads.js:1177)
    at Process.expectReport (threads.js:1161)
    at Process.evaluateContext (threads.js:779)
    at Process.runStep (threads.js:693)
    at threads.js:367
    at Array.forEach (<anonymous>)
    at ThreadManager.step (threads.js:365)
    at StageMorph.step (objects.js:8431)
    at StageMorph.Morph.stepFrame (morphic.js:3204)

thanks for the bug report!
I've just released a patch (v7.0.3) that addresses this (and some other) issues. You may need to perform a "hard reload" (reload Snap! in the browser while keeping the shift key down) or to flush your browser cache in order for the changes to become active right away.

No problem!