Jonas Jenwald 5bd95df427 Prevent TypeError: page is undefined when the document has been destroyed (PR 6546 follow-up)
*Follow-up to PR 6546.*

If rendering has already started when the document is destroyed, then `this.pageCache[data.pageIndex]` may already have been cleared when the `StartRenderingPage`/`RenderPageChunk` messages are recieved in `api.js`, which results in `TypeError`s being thrown.
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