I think this problem is not related to plasma or KDE and it is an internal problem with manjaro maybe.
In order to get logs from plasmashell, I ran following command:
$plasmashell --replace
This command replace your plasma and you are able to see logs from plasma in terminal.
When problem occurred again, there was no log or error in output of plasma in terminal.
Besides this, when system crashes, my keyboard doesn’t work anymore too (activing CapsLock fails) and this give me more hint that this problem is not related to plasma or it least is not limited to plasma.