JavaScript parse tree visitor: check if ctx.children is non-null in visitChildren

[robertbrignull]

I've been encountering a problem where the default generated visitor in javascript. It will crash because in the visit method it tries to call accept on null because it has been passed a null object by visitChildren.

This seems like a bug to me as the children field is expected to sometimes be null as it is initialised to null and only made non-null when a child is added. Other places handle this case explicitly but here it's assumed to be non-null.

Making the change in this PR makes my visitor work correctly. Do you think it is the right change to make, or if not what should I be doing instead?

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To explain my reasoning on why this change and not any other:

• The check could easily be ctx.getChildCount() > 0 and behave the same. I went for this as it's shorter and we're already accessing ctx.children directly instead of using getChild so it seems in keeping with the current code.
• The check could also have been done in visit by changing the else to else if (ctx) but this seemed a more intrusive way of fixing it and it's entirely reasonable for visit to expect a non-null argument.

[ericvergnaud]

Maybe add: else return null?

[robertbrignull]

Sure, will do

[ericvergnaud]

@parrt blessed