swift SDK crashed when call next page

> Please provide us with the following information:
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### This issue is for a: (mark with an `x`)
```
- [✔️] bug report -> please search issues before submitting
- [×] feature request
- [×] documentation issue or request
- [×] regression (a behavior that used to work and stopped in a new release)
```

### Minimal steps to reproduce
> swift listMessages, call next will crash (just next page is empty, It's very easy to reproduce).

var itar = list.syncIterator
while let msg = itar.next() {}

### Any log messages given by the failure
>

### Expected/desired behavior
>

### OS and Version?
> iOS

### Versions
> 1.2.0

### Mention any other details that might be useful

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