This error occurs when trying to close a world, which instead of the "save and exit" menu appears simply stays there on a "load" screen and it is impossible to carry out any other action.
It seemed to be resolved by uninstalling the application and reinstalling it, however when closing the game and reopening the problem reappeared, I tried it several times in the same way with the same result.
Attached link of a video where the failure is shown since here it does not allow me to upload the full video.
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We're tracking this issue in MCPE-78040, so this ticket is being resolved and linked as a duplicate.
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