The cause of this may be related to the following, just spotted in the system log, after logout:
kernel: [11865.258104] kactivitymanage[3507]: segfault at 7f7cb586acd0 ip 00007f7c9992a1b1 sp 00007ffef2dc4348 error 4 in libQt5Sql.so.5.4.2[7f7c99916000+3f000] Andy.
