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That means that program is using an index that is past the end of the array.
Remember that array indexes go from 0 to the array length-1
Make sure the index's value never goes past the array's...

Use the items on the current board to set the items on the next board.

Use the rules of the game to change the contents of a slot depending on what neighbors it has.
Perhaps you need two boards: current and next. Use the current board to set the xs in the next board. ...