Can't load document after error

[andjankodar]

Bug Report or Feature Request (mark with an x)

- [ ] Regression (a behavior that used to work and stopped working in a new release)
- [x] Bug report -> please search issues before submitting
- [ ] Feature request
- [ ] Documentation issue or request

Steps to reproduce:

1. Set the src to a document that can be loaded -> the lastLoaded variable gets the document url
2. Set the src to a document that fails to load -> the lastLoaded variable remains with the previous value
3. Set the src to the first document again -> the document is not loaded again as the src matches lastLoaded. However the _pdf variable has been cleared and an error occurs in get validPageNumbers

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