TextBuffer.getIterAtLineIndex

Obtains an iterator pointing to byte_index within the given line. byte_index must be the start of a UTF-8 character. Note bytes, not characters; UTF-8 may encode one character as multiple bytes.

Before the 3.20 version, it was not allowed to pass an invalid location.

Since the 3.20 version, if line_number is greater than the number of lines in the buffer, the end iterator is returned. And if byte_index is off the end of the line, the iterator at the end of the line is returned.