The game crashes while converting the world old world from MC-63070, namely: https://docs.google.com/file/d/0B57wgZSp2XofSXc4V1BLc1c5aU0/edit to the new version. Note that @unknown did not have this issue, see his comment.
MC-132382
Game crash while converting world
Resolved
Duplicate
0
Steven W.d.V.
(Unassigned)
Unconfirmed
conversion, convert, converter, converting, crash, crash-report
Minecraft 1.13-pre5
Retrieved
Also some mod should merge some of the tags I used for this issue; I mean "conversion convert converter converting"..? get real.. 😛