KoptInterface:getWordFromBoxes: guard against nil boxes

[pazos]

Fixes #6825

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This change is 

[NiLuJe]

Does this also prevent the stats issue or should that be taken care of, too?

[poire-z]

Does this also prevent the stats issue or should that be taken care of, too?

No, the stats issue is another thing to be taken care of.
I haven't given it much thoughts (as it's a bit of a mess, the initial initialization of things).
But at minima, adding to ReaderStatistics:onAddHighlight and ReaderStatistics:onDelHighlight a:
if not self.is_enabled then return end
shouldn't hurt, and avoid the error.

[NiLuJe]

@poire-z: Yup, I'll do that, thanks ;).