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At the time of writing the extension, there was something related to what functions have to be called/referenced for it to work on WebGL. If you have removed the lines and it still doesn’t fail, then you’re good.
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It would appear that the code split up between GMS1 and GMS2 versions so that would have to be ported manually. The error must have been due to argument count breaking change in GMS2.3.1 so the extension needs to be edited for that.
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You could get around this by changing the extension to return you base64 encoded string (canvas.toDataURL()) instead of saving and then passing it to sprite_add with help of another extension I made a while ago. This tutorial series may help.