I found a reason I think, for the latest loop step, Current node is set to null but the node is mandatory context for all internal array functions.

When/Where does it happen? I believe a proper fix would to avoid a dangling null node in the middle of eval, so if there is anything setting it to null, we should probably avoid setting null.

As I said it's not a completed code.

The exception throws when we are trying to get a result.
for ex:

var result = Template.Parse("{{['', '200', '','400'] | array.filter @string.strip}}").Evaluate(new TemplateContext());

var b = ((ICollection)result).Count; --> //bang!

Here:

I'm not sure what the correct behavior should be here. So I'm asking what else I can check.

I'm not sure what the correct behavior should be here. So I'm asking what else I can check.

That's what I suggested. Could you check when the node is going back to null? (e.g breakpoint/debug on the setter)

I think the issue is the same as explained in #322 - at least, the fix for that also allows the unit test in this PR to pass.

Fixed by changes in #324