This reproducible error occurs when trying to centre text, both by clicking the toolbar icon and by using ctrl-E. Tried in a new document and the same thing occurred. Crash report was automatically created and sent. The crash does not occur in 6.3.5.2
Thanks for reporting this issue. This is a duplicate of bug 130286 and it's fixed in LibreOffice 6.4.1.2, due to be released tomorrow *** This bug has been marked as a duplicate of bug 130286 ***