Prevent intermittent "Node was not found" errors when removing the `zoomLayer` in `PDFPageView_draw`
Prevent intermittent "Node was not found" errors when removing the `zoomLayer` in `PDFPageView_draw`

I've seen the above error occasionally when the scale is updated many times in quick succession, but I've not been able to pinpoint exactly why it happens.
Since the error isn't caught, this means that the `pageViewDrawCallback` function doesn't run to completion.

Unfortunately, given the very intermittent nature of the issue, I haven't got any good STR for reliably reproducing this issue. However, I hope that this patch can be accepted anyway, since it's simple and should help prevent unnecessary errors.

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