Well, the crash apparently only occured with Qt 5.13 or lower, as mentioned in bug#404518#c20 .
But Plasma 5.19 requires Qt 5.14 as minimum anyway, so it's no wonder why it's not reproducible with that version anymore.
Although I don't know about Qt 5.12.9, the last one I tried was 5.12.7 (as shipped in openSUSE Leap 15.2) where it did still crash. (until the kirigami change was reverted in the distribution package to avoid this crash)
Well, the crash apparently only occured with Qt 5.13 or lower, as mentioned in bug#404518#c20 .
But Plasma 5.19 requires Qt 5.14 as minimum anyway, so it's no wonder why it's not reproducible with that version anymore.
Although I don't know about Qt 5.12.9, the last one I tried was 5.12.7 (as shipped in openSUSE Leap 15.2) where it did still crash. (until the kirigami change was reverted in the distribution package to avoid this crash)